The Weather of British Columbia

Transcription

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The Weather
of
British Columbia
Graphic Area Forecast 31
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The Weather
of
British Columbia
Graphic Area Forecast 31
by
Ross Klock
John Mullock
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Copyright
Copyright © 2001 NAV CANADA. All rights reserved. No part of this document
may be reproduced in any form, including photocopying or transmission electronically to any computer, without prior written consent of NAV CANADA. The information contained in this document is confidential and proprietary to NAV CANADA and may not be used or disclosed except as expressly authorized in writing by
NAV CANADA.
Trademarks
Product names mentioned in this document may be trademarks or registered trademarks of their respective companies and are hereby acknowledged.
Relief Maps
Copyright © 2000. Government of Canada with permission from Natural Resources
Canada
Design and illustration by
Ideas in Motion
Kelowna, British Columbia
ph: (250) 717-5937
[email protected]
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LAKP-British Columbia
The Weather of British Columbia
Graphic Area Forecast 31 Pacific Region
Preface
For NAV CANADA’s Flight Service Specialists (FSS), providing weather briefings
to help pilots navigate through the day-to-day fluctuations in the weather is a critical
role. While available weather products are becoming increasingly more sophisticated,
and at the same time more easily understood, an understanding of local and regional
climatological patterns is essential to the effective performance of this role.
This British Columbia Local Area Knowledge Aviation Weather manual is one of
a series of six publications prepared by Meteorological Service of Canada (MSC) for
NAV CANADA. Each of the six manuals corresponds to a specific graphic forecast
area (GFA) Domain, with the exception of the Nunavut – Arctic manual that covers
two GFA Domains. These manuals form an important part of the training program
on local aviation weather knowledge for FSS working in the area and a useful tool in
the day-to-day service delivery by FSS.
Within the GFA domains, the weather shows strong climatological patterns controlled either by season or topography. This manual describes the Domain of the
GFACN31. This area offers beautiful open spaces for flying but can also provide
harsh flying conditions. As most pilots flying the region can attest, these variations in
weather can take place quiet abruptly. From the rocky coast to jagged mountain peaks,
local topography plays a key role in determining both the general climatology and
local flying conditions in a particular region.
This manual provides some insight on specific weather effects and patterns in this
area. While a manual cannot replace intricate details and knowledge of British
Columbia that FSS and experienced pilots of the area have acquired over the years, this
manual is a collection of that knowledge taken from interviews with local pilots, dispatchers, Flight Service Specialists and MSC personnel.
By understanding the weather and hazards in this specific area, FSS will be more
able to assist pilots to plan their flights in a safe and efficient manner. While this is
the manual’s fundamental purpose, NAV CANADA recognizes the value of the
information collected for pilots themselves. More and better information on weather
in the hands of pilots will always contribute to aviation safety. For that reason, the
manuals are being made available to NAV CANADA customers.
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Acknowledgements
This manual was made possible through funding by NAV CANADA, Flight
Information Centre project office.
NAV CANADA would like to thank The Meteorological Service of Canada
(MSC), both national and regional personnel, for working with us to compile the
information for each Graphic Area Forecast (GFA) domain, and present it in a userfriendly, professional format. Special thanks also go to meteorologists Ross Klock
and John Mullock, Mountain Weather Centre, Kelowna. Ross’s regional expertise has
been instrumental for the development of the Pacific GFA document while John’s
experience and efforts have ensured high quality and consistent material from
Atlantic to Pacific to Arctic.
This endeavour could not have been as successful without the contributions of
many people within the aviation community. We would like to thank all the
participants who provided information through interviews with MSC, including
flight service specialists, pilots, dispatchers, meteorologists and other aviation groups.
Their willingness to share their experiences and knowledge contributed greatly to the
success of this document.
Roger M. Brown
January, 2002
Readers are invited to submit any comments to:
NAV CANADA
Customer Service Centre
77 Metcalfe St.
Ottawa, Ontario
K1P 5L6
Toll free phone line: 1-800-876-4693-4
(within North America disregard the last digit)
Toll-free fax line: 1-877-663-6656
E-mail: [email protected]
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TABLE OF CONTENTS
PREFACE
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii
ACKNOWLEDGEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .iv
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ix
CHAPTER 1
BASICS OF METEOROLOGY . . . . . . . . . . . . . . . . . .1
Heat Transfer and Water Vapour . . . . . . . . . . . . . . . . . . .1
Lifting Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2
Subsidence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
Temperature Structure of the Atmosphere . . . . . . . . . . . .4
Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
Wind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
Air Masses and Fronts . . . . . . . . . . . . . . . . . . . . . . . . . . .6
CHAPTER 2
WEATHER HAZARDS TO AVIATION . . . . . . . . . . .9
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Icing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
The Freezing Process . . . . . . . . . . . . . . . . . . . . . . . . .10
Types of Aircraft Ice . . . . . . . . . . . . . . . . . . . . . . . . .10
Meteorological Factors Affecting Icing . . . . . . . . . . . .11
Aerodynamic Factors Affecting Icing . . . . . . . . . . . . .14
Other Forms of Icing . . . . . . . . . . . . . . . . . . . . . . . . .15
Visibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16
Types of Visibility . . . . . . . . . . . . . . . . . . . . . . . . . . .16
Causes of Reduced Visibility . . . . . . . . . . . . . . . . . . .17
Wind Shear and Turbulence . . . . . . . . . . . . . . . . . . . . . .19
Stability and the Diurnal Variation in Wind . . . . . . . .19
Wind Shear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
Relationship Between Wind, Shear and Turbulence . .21
Low Levels Jets - Frontal . . . . . . . . . . . . . . . . . . . . . .21
Low Levels Jets - Nocturnal . . . . . . . . . . . . . . . . . . . .22
Topographical Effects on Wind . . . . . . . . . . . . . . . . .22
Lee Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28
The Formation of Lee Waves . . . . . . . . . . . . . . . . . . .28
Characteristics of Lee Waves . . . . . . . . . . . . . . . . . . .29
Clouds Associated with Lee Waves . . . . . . . . . . . . . .30
Fronts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31
Frontal Weather . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32
Frontal Waves and Occlusions . . . . . . . . . . . . . . . . . .33
Thunderstorms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35
The Life Cycle of a Thunderstorm . . . . . . . . . . . . . . .36
Types of Thunderstorms . . . . . . . . . . . . . . . . . . . . . . .38
Thunderstorm Hazards . . . . . . . . . . . . . . . . . . . . . . .40
Severe Thunderstorms . . . . . . . . . . . . . . . . . . . . . . . .40
Cold Weather Operations . . . . . . . . . . . . . . . . . . . . . . . .43
Volcanic Ash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45
Deformation Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45
CHAPTER 3
WEATHER PATTERNS OF BRITISH COLUMBIA 49
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .49
South Coast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50
North Coast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52
Thompson-Okanagan . . . . . . . . . . . . . . . . . . . . . . . . . .53
Kootenays and Columbias . . . . . . . . . . . . . . . . . . . . . . .55
Central and Northern Interior . . . . . . . . . . . . . . . . . . . .57
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CHAPTER 4
Northeast British Columbia . . . . . . . . . . . . . . . . . . . . . .59
The Mean Atmospheric Circulation System . . . . . . . . .60
Upper Troughs and Upper Ridges . . . . . . . . . . . . . . . . .61
Semi-Permanent Surface Features . . . . . . . . . . . . . . . . .62
Migratory Surface Weather Systems . . . . . . . . . . . . . . .63
Winter Storms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .63
Gulf of Alaska Lows . . . . . . . . . . . . . . . . . . . . . . . . . . .64
Coastal Lows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .65
Winter Frontal Systems . . . . . . . . . . . . . . . . . . . . . . . . .66
Winter High Pressure Systems . . . . . . . . . . . . . . . . . . .66
Arctic Outbreaks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .67
Outflow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .68
Inflow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .68
Summer Weather . . . . . . . . . . . . . . . . . . . . . . . . . . . . .69
Summer Fronts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .69
Thermal Troughs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .70
Cold Lows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .71
SEASONAL WEATHER AND LOCAL EFFECTS .75
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .75
South Coast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .76
Local Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .80
East Coast of Vancouver Island – Victoria to Nanaimo .80
Vancouver Area Including Pitt Meadows, Langley
and Boundary Bay . . . . . . . . . . . . . . . . . . . . . . . . . . .82
Abbotsford to Hope . . . . . . . . . . . . . . . . . . . . . . . . .83
Vancouver to Pemberton along Howe Sound . . . . . . .85
Strait of Georgia – Vancouver - Nanaimo Powell River - Comox . . . . . . . . . . . . . . . . . . . . . . . .87
Inner Straits from Powell River/Comox –
Queen Charlotte Sound . . . . . . . . . . . . . . . . . . . . . .88
West Vancouver Island Including Routes to
Port Alberni and Tofino . . . . . . . . . . . . . . . . . . . . . . .91
North Coast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .93
Local Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .96
Northern Vancouver Island to McInnes Island . . . . . .96
Bella Bella to Prince Rupert . . . . . . . . . . . . . . . . . . . .98
Central Coast to the Interior Plateau . . . . . . . . . . . .100
Bella Bella to Kitimat (Douglas Channel) . . . . . . . . .102
Prince Rupert to Stewart . . . . . . . . . . . . . . . . . . . . .103
Queen Charlotte Islands . . . . . . . . . . . . . . . . . . . . .104
Prince Rupert to Terrace/Kitimat . . . . . . . . . . . . . .105
Routes Inland from Terrace . . . . . . . . . . . . . . . . . . .106
Thompson - Okanagan . . . . . . . . . . . . . . . . . . . . . . . . .107
Local Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .110
Routes Inland from the Coast . . . . . . . . . . . . . . . . .110
Cache Creek to Kamloops . . . . . . . . . . . . . . . . . . . .114
Princeton to Penticton . . . . . . . . . . . . . . . . . . . . . . .115
Okanagan Valley . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .116
Kamloops to Salmon Arm . . . . . . . . . . . . . . . . . . . .116
Kootenays and Columbias . . . . . . . . . . . . . . . . . . . . . . .118
Local Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .120
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Southern Route – Osoyoos to Cranbrook
and Eastwards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .120
Osoyoos to Castlegar . . . . . . . . . . . . . . . . . . . . . . . .121
Castlegar to Cranbrook . . . . . . . . . . . . . . . . . . . . . .123
Cranbrook and eastwards through the Crowsnest Pass 124
Northern Route – Salmon Arm to Banff . . . . . . . . . . .125
Salmon Arm to Revelstoke . . . . . . . . . . . . . . . . . . .125
Revelstoke to Golden to Banff . . . . . . . . . . . . . . . . .126
Rocky Mountain Trench . . . . . . . . . . . . . . . . . . . . . . .128
Cranbrook to Golden . . . . . . . . . . . . . . . . . . . . . . .129
Blaeberry Pass, Golden to the
North Saskatchewan Crossing . . . . . . . . . . . . . . . . .130
Golden - the Mica Dam – Tete Juane - Jasper . . . . .132
Central and Northern Interior . . . . . . . . . . . . . . . . . . .134
Local Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .136
Lillooet - Pavilion - Clinton . . . . . . . . . . . . . . . . . . .136
Clinton to Williams Lake . . . . . . . . . . . . . . . . . . . . .137
Williams Lake - Alexis Creek Puntzi Mountain - Anahim Lake . . . . . . . . . . . . . .138
Anahim Lake to the Coast . . . . . . . . . . . . . . . . . . .139
Williams Lake to Quesnel . . . . . . . . . . . . . . . . . . . .139
Quesnel to Prince George . . . . . . . . . . . . . . . . . . . .140
Kamloops - Vavenby - Blue River - Tete Jaune . . . . .141
Tete Jaune Cache - Mcbride - Prince George . . . . . .142
Prince George to Mackenzie . . . . . . . . . . . . . . . . . .143
Crossing the Rockies: Mackenzie to
Dawson Creek/Fort St. John . . . . . . . . . . . . . . . . . .144
Mackenzie to Watson Lake along
the Rocky Mountain Trench . . . . . . . . . . . . . . . . . . .146
Prince George to Smithers . . . . . . . . . . . . . . . . . . . .148
Smithers to Terrace via Telkwa Pass . . . . . . . . . . . . .149
Smithers to Terrace via the Bulkley / Skeena River
Valleys and the Shorter Route via the Kitseguecla
Valley . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .150
North from Terrace to the Nass River Valley . . . . . . .151
New Hazelton to Meziadin via the Kispiox River
Valley or the Kitwanga River Valley . . . . . . . . . . . . .152
Meziadin to Stewart . . . . . . . . . . . . . . . . . . . . . . . . .153
Meziadin to Bob Quinn Lake . . . . . . . . . . . . . . . . .154
Bob Quinn Lake to Dease Lake . . . . . . . . . . . . . . . .155
Dease Lake to Watson Lake . . . . . . . . . . . . . . . . . . .156
Dease Lake to Teslin . . . . . . . . . . . . . . . . . . . . . . . .157
Atlin north to the Yukon Border . . . . . . . . . . . . . . .158
Northeast British Columbia . . . . . . . . . . . . . . . . . . . . .159
Local effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .162
Dawson Creek to Fort Nelson . . . . . . . . . . . . . . . . .162
Fort Nelson to Watson Lake . . . . . . . . . . . . . . . . . .164
CHAPTER 5
Airport Climatology . . . . . . . . . . . . . . . . . . . . . . . . . . .167
GLOSSARY
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .201
TABLE OF SYMBOLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .207
APPENDIX
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .208
MAP INDEX
Chapter 4 Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .209
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Introduction
Meteorology is the science of the atmosphere, a sea of air that is in a constant state
of flux. Within it storms are born, grow in intensity as they sweep across sections of
the globe, then dissipate. No one is immune to the day-to-day fluctuations in the
weather, especially the aviator who must operate within the atmosphere.
Traditionally, weather information for the aviation community has largely been
provided in textual format. One such product, the area forecast (FA), was designed to
provide the forecast weather for the next twelve hours over a specific geographical
area. This information consisted of a description of the expected motion of significant
weather systems, the associated clouds, weather and visibility.
In April 2000, the Graphical Area Forecast (GFA) came into being, superceding
the area forecast. A number of MSC Forecast Centres now work together, using
graphical software packages, to produce a single national graphical depiction of the
forecast weather systems and the associated weather. This single national map is then
partitioned into a number of GFA Domains for use by Flight Service Specialists,
flight dispatchers and pilots.
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x
This Pacific Local Area Knowledge Aviation Weather Manual is one of a series of
six similar publications. All are produced by NAV CANADA in partnership with the
MSC. These manuals are designed to provide a resource for Flight Service Specialists
and pilots to help with the understanding of local aviation weather. Each of the six
manuals corresponds to a Graphical Area Forecast (GFA) Domain, with the exception of the Nunavut - Arctic manual which covers two GFA Domains. MSC aviation
meteorologists provide most of the broader scale information on meteorology and
weather systems affecting the various domains. Experienced pilots who work in or
around it on a daily basis, however, best understand the local weather. Interviews with
local pilots, dispatchers and Flight Service Specialists form the basis for the information presented in Chapter 4.
Within the domains, the weather shows strong climatological patterns that are
controlled either by season or topography. For example, in British Columbia there is
a distinctive difference between the moist coastal areas and the dry interior because
of the mountains. The weather in the Arctic varies strongly seasonally between the
frozen landscape of winter and the open water of summer. These changes are important in understanding how the weather works and each book will be laid out so as to
recognize these climatological differences.
This manual describes the weather of the GFACN31 Pacific. This area often has
beautiful flying weather but can also have some of the toughest flying conditions in
the world. Shifting dramatically from the dripping, fog-shrouded rain forests of the
West Coast, through the parched valley bottoms of the Interior, to the majestic snowcapped peaks and glaciers of the Rocky Mountains, few places in the world offer more
visual splendors for a pilot and passengers. At the same time, mountains also evoke
rapidly changing weather conditions that all too often have contributed to a tragedy.
Between 1976 and 1994, there were 419 flying accidents in British Columbia where
weather has been identified as one of the contributing factors. In these accidents, 319
people were killed and 89 people injured seriously. Mountain flying itself is not inherently dangerous, rather it is the weather associated with these areas that tends to be
unforgiving of the rash, the negligent and the unlucky.
This manual is “instant knowledge” about how the weather behaves in this area but
it is not “experience”. The information presented in this manual is by no means
exhaustive. The variability of local aviation weather in British Columbia could result
in a publication several times the size of this one. However, by understanding some
of the weather and hazards in these areas, pilots may be able to relate the hazards to
topography and weather systems in areas not specifically mentioned.
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Chapter 1
Basics of Meteorology
To properly understand weather, it is essential to understand some of the basic
principles that drive the weather machine. There are numerous books on the market
that describe these principles in great detail with varying degrees of success. This section is not intended to replace these books, but rather to serve as a review.
Heat Transfer and Water Vapour
The atmosphere is a “heat engine” that runs on one of the fundamental rules of
physics: excess heat in one area (the tropics) must flow to colder areas (the poles).
There are a number of different methods of heat transfer but a particularly efficient
method is through the use of water.
Within our atmosphere, water can exist in three states depending on its energy
level. Changes from one state to another are called phase changes and are readily
accomplished at ordinary atmospheric pressures and temperatures. The heat taken in
or released during a phase change is called latent heat.
LATENT HEAT ABSORBED
LATENT HEAT RELEASED
EL
TI
ON
TI
SA
NG
EN
ND
CO
AP
EV
M
ON
EE
ZI
I
AT
NG
OR
FR
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SUBLIMATION
DEPOSITION
Fig. 1-1 - Heat transfer and water vapour
How much water the air contains in the form of vapour is directly related to its
temperature. The warmer the air, the more water vapour it can contain. Air that contains its maximum amount of water vapour, at that given temperature, is said to be
saturated. A quick measure of the moisture content of the atmosphere can be made
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by looking at the dew point temperature. The higher (warmer) the dew point temperature, the greater the amount of water vapour.
The planetary heat engine consists of water being evaporated by the sun into water
vapour at the equator (storing heat) and transporting it towards the poles on the
winds where it is condensed back into a solid or liquid state (releasing heat). Most of
what we refer to as “weather,” such as wind, cloud, fog and precipitation is related to
this conversion activity. The severity of the weather is often a measure of how much
latent heat is released during these activities.
Lifting Processes
The simplest and most common way water vapour is converted back to a liquid or
solid state is by lifting. When air is lifted, it cools until it becomes saturated. Any
additional lift will result in further cooling which reduces the amount of water vapour
the air can hold. The excess water vapour is condensed out in the form of cloud
droplets or ice crystals which then can go on to form precipitation. There are several
methods of lifting an air mass. The most common are convection, orographic lift
(upslope flow), frontal lift, and convergence into an area of low pressure.
Fig. 1-2 - Convection as a result of
daytime heating
Fig.1-3 - Orographic (upslope) lift
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Fig.1-4 - Warm air overrunning cold air along a warm front
Surfac
rface Divergence
er
ce
Conve
onve
onvergence
Fig. 1-5 - Divergence and convergence at the surface and aloft
in a high low couplet
Subsidence
Subsidence, in meteorology, refers to the downward motion of air. This subsiding
motion occurs within an area of high pressure, as well as on the downward side of
a range of hills or mountains. As the air descends, it is subjected to increasing
atmospheric pressure and, therefore, begins to compress. This compression causes
the air’s temperature to increase which will consequently lower its relative humidity.
As a result, areas in which subsidence occurs will not only receive less precipitation
than surrounding areas (referred to as a “rain shadow”) but will often see the cloud
layers thin and break up.
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Fig.1-6 - Moist air moving over mountains where it loses its moisture and sinks
into a dry subsidence area
Temperature Structure of the Atmosphere
The temperature lapse rate of the atmosphere refers to the change of temperature
with a change in height. In the normal case, temperature decreases with height
through the troposphere to the tropopause and then becomes relatively constant in the
stratosphere.
Two other conditions are possible: an inversion, in which the temperature increases with height, or an isothermal layer, in which the temperature remains constant with
height.
-9°C
NORMAL
A
L
T
I
T
U
D
E
3°C
ISOTHERMAL
LAYER
3°C
INVERSION
-2°C
Fig. 1-7 - Different lapse rates of the atmosphere
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The temperature lapse rate of the atmosphere is a direct measurement of the stability of the atmosphere.
Stability
It would be impossible to examine weather without taking into account the stability of the air. Stability refers to the ability of a parcel of air to resist vertical motion.
If a parcel of air is displaced upwards and then released it is said to be unstable if it
continues to ascend (since the parcel is warmer than the surrounding air), stable if it
returns to the level from which it originated (since the parcel is cooler than the surrounding air), and neutral if the parcel remains at the level it was released (since the
parcel’s temperature is that of the surrounding air).
The type of cloud and precipitation produced varies with stability. Unstable air,
when lifted, has a tendency to develop convective clouds and showery precipitation.
Stable air is inclined to produce deep layer cloud and widespread steady precipitation.
Neutral air will produce stable type weather which will change to unstable type
weather if the lifting continues.
(a)
(b)
(c)
Fig. 1-8 - Stability in the atmosphere - (a) Stable (b) Unstable (c) Neutral
The stability of an air mass has the ability to be changed. One way to destabilize
the air is to heat it from below, in much the same manner as you would heat water in
a kettle. In the natural environment this can be accomplished when the sun heats the
ground which, in turn, heats the air in contact with it, or when cold air moves over a
warmer surface such as open water in the fall or winter. The reverse case, cooling the
air from below, will stabilize the air. Both processes occur readily.
Consider a typical summer day where the air mass is destabilized by the sun, resulting in the development of large convective cloud and accompanying showers or thundershowers during the afternoon and evening. After sunset, the surface cools and the
air mass stabilizes slowly, causing the convective activity to die off and the clouds to
dissipate.
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On any given day there may be several processes acting simultaneously that can
either destabilize or stabilize the air mass. To further complicate the issue, these competing effects can occur over areas as large as an entire GFA domain to as small as a
football field. To determine which one will dominate remains in the realm of a meteorologist and is beyond the scope of this manual.
Wind
Horizontal differences in temperature result in horizontal differences in pressure. It
is these horizontal changes in pressure that cause the wind to blow as the atmosphere
attempts to equalize pressure by moving air from an area of high pressure to an area
of low pressure. The larger the pressure difference, the stronger the wind and, as a
result, the day-to-day wind can range from the gentlest breeze around an inland airfield to storm force winds over the water.
Pressure difference
from A to B is 4 hPa
in 70 miles
Pressure difference
from C to D is 4 hPa
in 200 miles
Fig. 1-9 - The greater pressure changes with horizontal difference, the stronger the wind
Wind has both speed and direction, so for aviation purposes several conventions
have been adopted. Wind direction is always reported as the direction from which the
wind is blowing while wind speed is the average steady state value over a certain
length of time. Short-term variations in speed are reported as either gusts or squalls
depending on how long they last.
Above the surface, the wind tends to be relatively smooth and changes direction
and speed only in response to changes in pressure. At the surface, however, the wind
is affected by friction and topography. Friction has a tendency to slow the wind over
rough surfaces whereas topography, most commonly, induces localized changes in
direction and speed.
Air Masses and Fronts
Air Masses
When a section of the troposphere, hundreds of miles across, remains stationary or
moves slowly across an area having fairly uniform temperature and moisture, then the
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air takes on the characteristics of this surface and becomes known as an air mass. The
area where air masses are created are called “source regions” and are either ice or snow
covered polar regions, cold northern oceans, tropical oceans or large desert areas.
Although the moisture and temperature characteristics of an air mass are relatively
uniform, the horizontal weather may vary due to different processes acting on it. It is
quite possible for one area to be reporting clear skies while another area is reporting
widespread thunderstorms.
Fronts
When air masses move out of their source regions they come into contact with
other air masses. The transition zone between two different air masses is referred to
as a frontal zone, or front. Across this transition zone temperature, moisture content,
pressure, and wind can change rapidly over a short distance.
The principal types of fronts are:
Cold Front - The cold air is advancing and undercutting the warm air.
The leading edge of the cold air is the cold front.
Warm front - The cold air is retreating and being replaced by warm
air. The trailing edge of the cold air is the warm front.
Stationary front - The cold air is neither advancing nor retreating.
These fronts are frequently referred to quasi-stationary fronts although
there usually is some small-scale localized motion occurring.
Trowal - Trough of warm air aloft.
Table 1-1
More will be said about frontal weather later in this manual.
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Chapter 2
Aviation Weather Hazards
Introduction
Throughout its history, aviation has had an intimate relationship with the weather.
Time has brought improvements - better aircraft, improved air navigation systems
and a systemized program of pilot training. Despite this, weather continues to exact
its toll.
In the aviation world, ‘weather’ tends to be used to mean not only “what’s happening now?” but also “what’s going to happen during my flight?”. Based on the answer
received, the pilot will opt to continue or cancel his flight. In this section we will
examine some specific weather elements and how they affect flight.
Icing
One of simplest assumptions made about clouds is that cloud droplets are in a
liquid form at temperatures warmer than 0°C and that they freeze into ice crystals
within a few degrees below zero. In reality, however, 0°C marks the temperature below
which water droplets become supercooled and are capable of freezing. While some
of the droplets actually do freeze spontaneously just below 0°C, others persist in the
liquid state at much lower temperatures.
Aircraft icing occurs when supercooled water droplets strike an aircraft whose
temperature is colder than 0°C. The effects icing can have on an aircraft can be quite
serious and include:
Lift Decreases
Drag
Increases
Stall Speed
Increases
Weight Increases
Fig. 2-1 - Effects of icing
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• disruption of the smooth laminar flow over the wings causing a decrease in lift
and an increase in the stall speed. This last effect is particularly dangerous. An
“iced” aircraft is effectively an “experimental” aircraft with an unknown stall
speed.
• increase in weight and drag thus increasing fuel consumption.
• partial or complete blockage of pitot heads and static ports giving erroneous
instrument readings.
• restriction of visibility as windshield glazes over.
The Freezing Process
When a supercooled water droplet strikes an aircraft surface, it begins to freeze,
releasing latent heat. This latent heat warms the remainder of the droplet to near 0°C,
allowing the unfrozen part of the droplet to spread back across the surface until freezing is complete. The lower the air temperature and the colder the aircraft surface, the
greater the fraction of the droplet that freezes immediately on impact. Similarly, the
smaller the droplet, the greater the fraction of the droplet that freezes immediately on
impact. Finally, the more frequent the droplets strike the aircraft surface, the greater
the amount of water that will flow back over the aircraft surface. In general, the maximum potential for icing occurs with large droplets at temperatures just below 0°C.
Fig. 2-2 - Freezing of supercooled droplets on impact
Types of Aircraft Ice
Rime Ice
Rime ice is a product of small droplets where each droplet has a chance to freeze
completely before another droplet hits the same place. The ice that is formed is
opaque and brittle because of the air trapped between the droplets. Rime ice tends to
form on the leading edges of airfoils, builds forward into the air stream and has low
adhesive properties.
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Clear Ice
In the situation where each large droplet does not freeze completely before additional droplets become deposited on the first, supercooled water from each drop
merges and spreads backwards across the aircraft surface before freezing completely
to form an ice with high adhesive properties. Clear ice tends to range from transparent to a very tough opaque layer and will build back across the aircraft surface as well
as forward into the air stream.
Mixed Ice
When the temperature and the range of droplet size vary widely, the ice that forms
is a mixture of rime ice and clear ice. This type of ice usually has more adhesive properties than rime ice, is opaque in appearance, rough, and generally builds forward into
the air stream faster than it spreads back over the aircraft surface.
Clear
Rime
Mixed
Fig. 2-3 - Accumulation patterns of different icing types
Meteorological Factors Affecting Icing
(a) Liquid Water Content of the Cloud
The liquid water content of a cloud is dependent on the size and number of
droplets in a given volume of air. The greater the liquid water content, the more
serious the icing potential. Clouds with strong vertical updrafts generally have a
higher liquid water content as the updrafts prevent even the large drops from precipitating.
The strongest updrafts are to be found in convective clouds, clouds formed by
abrupt orographic lift, and in lee wave clouds. Layer clouds tend to have weak
updrafts and are generally composed of small droplets.
(b) Temperature Structure in the Cloud
Warm air can contain more water vapour than cold air. Thus, clouds that form in
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warm air masses will have a higher liquid water content than those that form in
cold air.
The temperature structure in a cloud has a significant effect on the size and number of droplets. Larger supercooled droplets begin to freeze spontaneously around
-10°C with the rate of freezing of all size of droplets increasing rapidly as temperatures fall below -15°C. By -40°C, virtually all the droplets will be frozen. The
exceptions are clouds with very strong vertical updrafts, such as towering cumulus
or cumulonimbus, where liquid water droplets can be carried to great heights before
freezing.
These factors allow the icing intensities to change rapidly with time so that it is
possible for aircraft only minutes apart to encounter entirely different icing conditions
in the same area. Despite this, some generally accepted rules have been developed:
Ice Crystals
-40C
Layer
Cloud
Supercooled
Water Droplets
and
Ice Crystals
Supercooled
Water Droplets
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-40C
Convective
Cloud
-15C
-10C
0C
-15C
Mostly
Supercooled
Water Droplets
-10C
Water Droplets
0C
Fig. 2-4 - Distribution of water droplet-ice crystals in cloud
(1) Within large cumulus and cumulonimbus clouds:
• at temperatures between 0°C and -25°C, severe clear icing likely.
• at temperatures between -25°C and -40°C, light rime icing likely; small possibility of moderate to severe rime or mixed icing in newly developed clouds.
• at temperatures below -40°C, little chance of icing.
(2) Within layer cloud:
• the most significant icing layer is generally confined to the 0°C to -15°C temperature range.
• icing is usually less severe than in convective cloud due to the weaker updrafts
and smaller droplets.
• icing layers tend to be shallow in depth but great in horizontal extent.
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(3) Situations in which icing may be greater than expected:
• air moving across large unfrozen lakes in the fall and winter will increase its
moisture content and destabilize rapidly due to heating from below. The cloud
that forms, while resembling a layer cloud, will actually be a convective cloud
capped by an inversion with relatively strong updrafts and a large concentration
of supercooled drops.
• thick layer cloud formed by rapid mass ascent, such as in an intensifying low or
along mountain slopes, will also have enhanced concentrations of supercooled
drops. Furthermore, there is a strong possibility that such lift will destabilize the
air mass resulting in embedded convective clouds with their enhanced icing
potential.
• lenticular clouds can have very strong vertical currents associated with them.
Icing can be severe and, because of the droplet size, tend toward clear icing.
Supercooled Large Drop Icing
Supercooled large drop (SLD) icing has, until fairly recently, only been associated
with freezing rain. Several accidents and significant icing events have revealed the
existence of a deadly form of SLD icing in non-typical situations and locations. It was
found that large cloud drops, the size of freezing drizzle drops, could exist within
some stratiform cloud layers, whose cloud top is usually at 10,000 feet or less. The air
temperature within the cloud (and above) remains below 0°C but warmer than -18°C
throughout the cloud layer. These large drops of liquid water form near the cloud top,
in the presence of light to moderate mechanical turbulence, and remain throughout
the cloud layer. SLD icing is usually severe and clear. Ice accretion onto flight surfaces of 2.5 cm or more in 15 minutes or less have been observed.
There are a few indicators that may help announce SLD icing beforehand. SLD
icing-producing stratiform clouds often occur in a stable air mass, in the presence of
a gentle upslope circulation, sometimes coming from a large body of water. The air
above the cloud layer is always dry, with no significant cloud layers above. The presence of freezing drizzle underneath, or liquid drizzle when the surface air temperature is slightly above 0°C, is a sure indication of SLD icing within the cloud. Other
areas where this type of icing is found is in the cloud to the southwest of a low pressure centre and behind cold fronts where low level stratocumulus are common (cloud
tops often below 13,000 feet). Constant and careful attention must be paid when flying a holding pattern within a cloud layer in winter.
Over British Columbia, SLD icing-producing clouds are common along the coast
with a westerly flow and in the valleys, such as the Okanagan, where they tend to
cover the whole valley and lie near the mountain tops. These low-level clouds often
produce drizzle or freezing drizzle.
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The Glory: A Warning Sign for Aircraft Icing
Photo 2-1 - Glory surrounding aircraft shadow
on cloud top.
credit: Alister Ling
The glory is one of the most common forms of halo visible in the sky. For the pilot
it is a warning sign of potential icing because it is only visible when there are liquid
water droplets in the cloud. If the air temperature at cloud level is below freezing,
icing will occur in those clouds that produce a glory.
A glory can be seen by looking downwards and seeing it surround the shadow that
your aircraft casts onto the cloud tops. They can also be seen by looking upwards
towards the sun (or bright moon) through clouds made of liquid droplets.
It is possible to be high enough above the clouds or fog that your shadow is too
small to see at the center of the glory. Although ice crystals often produce other halos
and arcs, only water droplets form bullseyes.
Aerodynamic Factors Affecting Icing
There are various aerodynamic factors that affect the collection efficiency of an aircraft surface. Collection efficiency can be defined as the fraction of liquid water
droplets that actually strike the aircraft relative to the number of droplets encountered
along the flight path.
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Larger Droplets
150 KT
150 KT
(a)
Airfoil Shape
150 KT
100 KT
150 KT
160 KT
(b)
Aircraft Speed
(c)
Droplet Size
Fig. 2-5 -Variations in collection efficiency
Collection efficiency is dependent on three factors:
(a) The radius of curvature of the aircraft component. Airfoils with a big radius of
curvature disrupt the airflow (like a bow wave) causing the smaller supercooled
droplets to be carried around the airfoil by the air stream. For this reason, large
thick components (thick wings, canopies) collect ice less efficiently than thin
components (thin wings, struts, antenna).
(b) Speed. The faster the aircraft the less chance the droplets have to be diverted
around the airfoil by the air stream.
(c) Droplet size. The larger the droplet the more difficult it is for the air stream to
displace it.
Other Forms of Icing
(a) Freezing Rain and Ice Pellets
Freezing rain occurs when liquid water drops that are above freezing fall into a
layer of air whose temperature is colder than 0°C and supercool before hitting
some object. The most common scenario leading to freezing rain in British
Columbia is “warm overrunning”. In this case, warm air (above 0°C) is forced
up and over colder air at the surface. In such a scenario, rain that falls into the
cold air supercools, resulting in freezing rain that can last for hours especially if
cold air continues to drain into the area from the surrounding terrain. When
the cold air is sufficiently deep, the freezing raindrops can freeze completely
before reaching the surface causing ice pellets. Pilots should be aware, however,
that ice pellets at the surface imply freezing rain aloft. Such conditions are relatively common in the winter and tend to last a little longer in valleys than over
flat terrain.
(b) Freezing Drizzle or Snow Grains
Freezing drizzle is different from freezing rain in that the water droplets are
smaller. Another important difference is that freezing drizzle may develop in air
masses whose entire temperature profile is below freezing. In other words,
freezing drizzle can occur without the presence of a warm layer (above 0°C)
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aloft. In this case, favorable areas for the development of freezing drizzle are in
moist maritime air masses, preferably in areas of moderate to strong upslope
flow. The icing associated with freezing drizzle may have a significant impact
on aviation. Similar to ice pellets, snow grains imply the presence of freezing
drizzle aloft.
(c) Snow
Dry snow will not adhere to an aircraft surface and will not normally cause
icing problems. Wet snow, however, can freeze hard to an aircraft surface that is
at subzero temperatures and be extremely difficult to remove. A very dangerous
situation can arise when an aircraft attempts to take off with wet snow on the
flight surfaces. Once the aircraft is set in motion, evaporational cooling will
cause the wet snow to freeze hard causing a drastic reduction in lift as well as
increasing the weight and drag. Wet snow can also freeze to the windscreens
making visibility difficult to impossible.
(d) Freezing Spray
Freezing spray develops over open water when there is an outbreak of Arctic
air. While the water itself is near or above freezing, any water that is picked up
by the wind or is splashed onto an object will quickly freeze, causing a rapid
increase in weight and shifting the centre of gravity.
(e) Freezing Fog
Freezing fog is a common occurrence during the winter. Fog is simply “a cloud
touching the ground” and, like its airborne cousin, will have a high percentage
of supercooled water droplets at temperatures just below freezing (0°C to -10°C).
Aircraft landing, taking off, or even taxiing, in freezing fog should anticipate
rime icing.
Visibility
Reduced visibility is the meteorological component which impacts flight operations
the most. Topographic features all tend to look the same at low levels making good
route navigation essential. This can only be done in times of clear visibility.
Types of Visibility
There are several terms used to describe the different types of visibility used by the
aviation community.
(a) Horizontal visibility - the furthest visibility obtained horizontally in a specific
direction by referencing objects or lights at known distances.
(b) Prevailing visibility - the ground level visibility which is common to one-half
or more of the horizon circle.
(c) Vertical visibility - the maximum visibility obtained by looking vertically
upwards into a surface-based obstruction such as fog or snow.
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(d) Slant visibility - visibility observed by looking forward and downwards from
the cockpit of the aircraft.
(e) Flight visibility - the average range of visibility at any given time forward from
the cockpit of an aircraft in flight.
Causes of Reduced Visibility
(a) Lithometers
Lithometers are dry particles suspended in the atmosphere and include haze,
smoke, sand and dust. Of these, smoke and haze cause the most problems. The
most common sources of smoke are forest fires. Smoke from distant sources
will resemble haze but, near a fire, smoke can reduce the visibility significantly.
(b) Precipitation
Rain can reduce visibility, however, the restriction is seldom less than one mile
other than in the heaviest showers beneath cumulonimbus clouds. Drizzle,
because of the greater number of drops in each volume of air, is usually more
effective than rain at reducing the visibility, especially when accompanied by fog.
Snow affects visibility more than rain or drizzle and can easily reduce it to less
than one mile. Blowing snow is a product of strong winds picking up the snow
particles and lifting them into the air. Fresh fallen snow is easily disturbed and
can be lifted a few hundred feet. Under extreme conditions, the cockpit visibility will be excellent during a landing approach until the aircraft flares, at which
time the horizontal visibility will be reduced abruptly.
(c) Fog
Fog is the most common and persistent visibility obstruction encountered by
the aviation community. A cloud based on the ground, fog, can consist of water
droplets, supercooled water droplets, ice crystals or a mix of supercooled
droplets and ice crystals.
(i) Radiation Fog
Radiation fog begins to form over land usually under clear skies and light
winds typically after midnight and peaks early in the morning. As the land
surface loses heat and radiates it into space, the air above the land is cooled
and loses its ability to hold moisture. If an abundance of condensation
nuclei is present in the atmosphere, radiation fog may develop before the
temperature-dewpoint spread reaches zero. After sunrise, the fog begins to
burn off from the edges over land but any fog that has drifted over water
will take longer to burn off.
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Photo 2-2 - Radiation fog in a valley
credit: Alister Ling
(ii) Precipitation or Frontal Fog
Precipitation fog, or frontal fog, forms ahead of warm fronts when precipitation falls through a cooler layer of air near the ground. The precipitation
saturates the air at the surface and fog forms. Breaks in the precipitation
usually results in the fog becoming thicker.
(iii) Steam Fog
Steam fog forms when very cold arctic air moves over relatively warmer
water. In this case moisture evaporates from the water surface and saturates
the air. The extremely cold air cannot hold all the evaporated moisture, so
the excess condenses into fog. The result looks like steam or smoke rising
from the water, and is usually no more than 50 to 100 feet thick. Steam fog,
also called arctic sea smoke, can produce significant icing conditions.
(iv) Advection Fog
Fog that forms when warm moist air moves across a snow, ice or cold water
surface.
(v) Ice Fog
Ice fog occurs when water vapour sublimates directly into ice crystals. In conditions of light winds and temperatures colder than -30°C or so, water vapour
from manmade sources or cracks in ice-covered rivers can form widespread
and persistent ice fog. The fog produced by local heating systems, and even
aircraft engines, can reduce the local visibility to near zero, closing an airport
for hours or even days.
(d) Snow Squalls and Streamers
Snow squalls are relatively small areas of heavy snowfall. They develop when
cold arctic air passes over a relatively warm water surface, such as Williston
Lake , before freeze-up. An injection of heat and moisture from the lake into
the low levels of the atmosphere destabilizes the air mass. If sufficient destabi-
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lization occurs, convective clouds begin to develop with snow beginning shortly
thereafter. Snowsqualls usually develop in bands of cloud, or streamers, that
form parallel to the direction of flow. Movement of these snow squalls can generally be tied to the mean winds between 3,000 and 5,000 feet. Not only
can snowsqualls reduce visibility to near zero but, due to their convective nature,
significant icing and turbulence are often encountered within the clouds.
Cold Air
Warm Water
Fig. 2-6 - Snowsqualls building over open water
Wind, Shear and Turbulence
The “why” of winds are quite well understood. It is the daily variations of the winds,
where they blow and how strong, that remains a constant problem for meteorologists
to unravel. The problem becomes even more difficult when local effects such as wind
flow through coastal inlets or in mountain valleys are added to the dilemma. The
result of these effects can give one airport persistent light winds while another has
nightly episodes of strong gusty winds.
Stability and the Diurnal Variation in Wind
In a stable weather pattern, daytime winds are generally stronger and gustier than
nighttime winds. During the day, the heating from the sun sets up convective mixing
which carries the stronger winds aloft down to the surface and mixes them with the
slower surface winds. This causes the surface wind to increase in speed and become
gusty, while at the same time reducing the wind speeds aloft in the mixed layer.
After sunset, the surface of the earth cools which, in turn, cools the air near the surface resulting in the development of a temperature inversion. This inversion deepens
as cooling continues, ending the convective mixing and causing the surface winds to
slacken.
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Wind Shear
Wind shear is nothing more than a change in wind direction and/or wind speed
over the distance between two points. If the points are in a vertical direction then it is
called vertical shear, if they are in a horizontal direction than it is called horizontal shear.
In the aviation world, the major concern is how abruptly the change occurs. If the
change is gradual, a change in direction or speed will result in nothing more than a
minor change in the ground speed. If the change is abrupt, however, there will be a
rapid change of airspeed or track. Depending on the aircraft type, it may take a significant time to correct the situation, placing the aircraft in peril, particularly during
takeoff and landing.
Significant shearing can occur when the surface wind blowing along a valley varies
significantly from the free flowing wind above the valley. Changes in direction of 90°
and speed changes of 25 knots are reasonably common in mountainous terrain.
Fig. 2-7 - Wind shear near the top of a valley
Updrafts and downdrafts also induce shears. An abrupt downdraft will cause a brief
decrease in the wing’s attack angle resulting in a loss of lift. An updraft will increase
the wing’s attack angle and consequently increase the lift, however, there is a risk that
it could be increased beyond the stall angle.
Shears can also be encountered along fronts. Frontal zones are generally thick
enough that the change is gradual, however, cold frontal zones as thin as 200 feet have
been measured. Significant directional shears across a warm front have also been
observed with the directional change greater than 90 degrees over several hundred
feet. Pilots doing a take-off or a landing approach through a frontal surface that is just
above the ground should be wary.
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Mechanical turbulence is a form of shear induced when a rough surface disrupts the
smooth wind flow. The amount of shearing and the depth of the shearing layer depends
on the wind speed, the roughness of the obstruction and the stability of the air.
The Relationship Between Wind Shear and Turbulence
Turbulence is the direct result of wind shear. The stronger the shear the greater the
tendency for the laminar flow of the air to break down into eddies resulting in turbulence. However, not all shear zones are turbulent, so the absence of turbulence does
not infer that there is no shear.
Low-Level Jets - Frontal
In developing low pressure systems, a narrow band of very strong winds often
develops just ahead of the cold front and above the warm frontal zone. Meteorologists
call these bands of strong winds “low-level jets”. They are typically located between
500 and 5,000 feet and can be several hundred feet wide. Wind speeds associated with
low-level jets can reach as high as 100 knots in more intense storms. The main
problem with these features is that they can produce severe turbulence, or at least
significant changes in airspeed. Critical periods for low-level windshear or turbulence
with these features are one to three hours prior to a cold frontal passage. These conditions are made worse by the fact that they occur in the low levels of the atmosphere
and affect aircraft in the more important phases of flight - landing and take off.
EV
E
JE
T
UP E R L E V E
LO
W
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WARM AIR
Fig. 2-8 - Idealized low and frontal system show the position
of the low-level and upper-level jet
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W
LO
VE
LE
L
JE
T
Fig. 2-9 - Complex winds around low level jet can result in
significant low-level wind shear and turbulence
Low-Level Jets - Nocturnal
There is another type of low-level jet known as “the low-level nocturnal jet”. This
jet is a band of relatively high wind speeds, typically centred at altitudes ranging
between 700 and 2,000 feet above the ground (just below the top of the nocturnal
inversion) but on occasion can be as high as 3,000 feet. Wind speeds usually range
between 20 and 40 knots but have been observed up to 60 knots.
Low-level nocturnal jets have been observed in mountainous terrain but tend to be
localized in character. The Central Interior of BC is a favoured location for these
events.
The low-level nocturnal jet forms mainly in the summer on clear nights (this allows
the inversion to form). The winds just below the top of the inversion will begin to
increase just after sunset, reach its maximum speed a couple of hours after midnight,
then dissipate in the morning as the sun’s heat destroys the inversion.
Topographical Effects on Wind
(a) Lee Effects
When the winds blow against a steep cliff or over rugged terrain, gusty turbulent winds result. Eddies often form downwind of the hills, which create stationary zones of stronger and lighter winds. These zones of strong winds are
fairly predictable and usually persist as long as the wind direction and stability
of the air stream do not change. The lighter winds, which occur in areas called
wind shadows can vary in speed and direction, particularly downwind of higher
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hills. In the lee of the hills, the wind is usually gusty and the wind direction is
often completely opposite to the wind blowing over the top of the hills. Smaller
reverse eddies may also be encountered close to the hills.
Fig. 2-10 - Lee effects
(b) Friction Effects
The winds that blow well above the surface of the earth are not strongly influenced by the presence of the earth itself. Closer to the earth, however, frictional
effects decrease the speed of the air movement and back the wind (turns the
wind direction counter-clockwise) towards the lower pressure. For example, in
the northern hemisphere, a southerly wind becomes more southeasterly when
blowing over rougher ground. There can be a significant reduction in the wind
speed over a rough terrain when compared to the wind produced by the same
pressure gradient over a relatively smooth prairie.
Fig. 2-11 - Friction effects
(c) Converging Winds
When two or more winds flow together or converge, a stronger wind is created.
Similar effects can be noted where two or more valleys come together.
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CHAPTER TWO
10 KT
15 KT
25
KT
Fig. 2-12 - Converging winds
(d) Diverging Winds
A divergence of the air stream occurs when a single air stream splits into two or
more streams. Each will have a lower speed than the parent air stream.
25 KT
15
KT
10
KT
Fig. 2-13 - Diverging winds
(e) Corner Winds
When the prevailing wind encounters a headland, there is a tendency for the
wind to curl around the feature. This change in direction, if done abruptly, can
result in turbulence.
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Fig. 2-14 - Corner winds
(f ) Funnelled or Gap Winds
When winds are forced to flow through a narrow opening or gap, such as an
inlet or narrow section of a pass, the wind speed will increase and may even
double in strength. This effect is similar to pinching a water hose and is called
funnelling.
Fig. 2-15 - Funnelled winds
(g) Channelled Winds
The topography can also change the direction of the winds by forcing the
flow along the direction of a pass or valley. This is referred to as channelling.
(h) Sea and Land Breezes
Sea and land breezes are only observed under light wind conditions, and
depend on temperature differences between adjoining regions.
A sea breeze occurs when the air over the land is heated more rapidly than the
air over the adjacent water surface. As a result, the warmer air rises and the rel-
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CHAPTER TWO
atively cool air from the water flows onshore to replace it. By late afternoon,
the time of maximum heating, the sea breeze circulation may be 1,500 to 3,000
feet deep, have obtained speeds of 10 to 15 knots and extend as far as 50 nautical miles inland.
Fig. 2-16 - Sea breeze
During the evening the sea breeze subsides. At night, as the land cools, a land
breeze develops in the opposite direction and flows from the land out over the
water. It is generally not as strong as the sea breeze, but at times it can be quite
gusty.
Fig. 2-17 - Land breeze
Both sea and land breezes can be influenced by channelling and funnelling
resulting in almost frontal-like conditions, with sudden wind shifts and gusty
winds that may reach up to 50 knots. Example of this can be found near the
larger lakes in BC are often referred to as “lake effect winds”.
(i) Anabatic and Katabatic Winds
During the day, the sides of the valleys become warmer than the valley bottoms
since they are better exposed to the sun. As a result, the winds blow up slope.
These daytime, upslope winds are called anabatic winds. Gently sloped valley
sides, especially those facing south, are more efficiently heated than those of a
steep, narrow valley. As a result, valley breezes will be stronger in the wider valleys. An anabatic wind, if extended to sufficient height, will produce cloud. In
addition, such a wind offers additional lift to aircraft and gliders.
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Fig. 2-18 - Anabatic winds
At night, the air cools over the mountain slopes and sinks to the valley floor. If the
valley floor is sloping, the winds will move along the valley towards lower ground.
The cool night winds are called drainage winds, or katabatic winds, and are often
quite gusty and usually stronger than anabatic winds. Some valley airports have
windsocks situated at various locations along their runways to show the changeable
conditions due to the katabatic flow.
Fig. 2-19 - Katabatic winds
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(j) Glacier Winds
Under extreme cooling conditions, such as an underlying ice cover, the katabatic winds can develop to hazardous proportions. As the ice is providing the cooling, a shallow wind of 80 knots or more can form and will persist during the
day and night. In some locations the katabatic flow “pulsates” with the cold air
building up to some critical value before being released to rush downslope.
Fig. 2-20 - Glacier winds
It is important to recognize that combinations of these effects can operate at any
given time. Katabatic winds are easily funnelled resulting in winds of unexpected
directions and strengths in narrow passes. Around glaciers in the summer, wind fields
can be chaotic as katabatic winds from the top of the glacier struggle for dominance
with localized convection or anabatic winds induced by heated rock slopes below the
ice. Many sightseeing pilots prefer to avoid glaciated areas during the afternoon hours.
Lee Waves
When air flows across a mountain or hill, it is disturbed the same way as water
flowing over a rock. The air initially is displaced upwards across the mountain, dips
sharply on the lee side, then rises and falls in a series of waves downstream. These
waves are called “mountain waves” or “lee waves” and are most notable for their turbulence. They often develop on the lee side of the Rocky Mountains.
The Formation of Lee Waves
The development of lee waves requires that several conditions be met:
(a) the wind direction must be within 30 degrees of perpendicular to the mountain
or hill. The greater the height of the mountain and the sharper the drop off to
the lee side, the more extensive the induced oscillations.
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(b) wind speed should exceed 15 knots
for small hills and 30 knots for mountain ridges. A jet stream with its associated strong winds below the jet axis
is an ideal situation.
(c) the wind direction should be constant
while increasing in speed with height
throughout the troposphere.
(d) the air should be stable near the
mountain peaks but less stable below.
The unstable layer encourages the air
to ascend and the stable layer encourages the development of a downstream wave pattern.
30
0
30
0
Fig. 2-21 - Angles for lee wave
development
While all these conditions can be met at any time of the year, winter wind speeds
are generally stronger resulting in more dangerous lee waves.
Characteristics of Lee Waves
Once a lee wave pattern has been established, it follows several basic rules:
• stronger the wind, the longer the wavelength. The typical wavelength is about 6
miles but can vary from as short as 3 miles to as long as 15 miles.
• position of the individual wave crests will remain nearly stationary with the
wind blowing through them as long as the mean wind speed remains nearly
constant.
• individual wave amplitude can exceed 3,000 feet.
• layer of lee waves often extends from just below the tops of the mountains to
4,000 to 6,000 feet above the tops but can extend higher.
• induced vertical currents within the wave can reach values of 4,500 feet per
minute.
• wind speed is stronger through the wave crest and slower through the wave
trough.
• wave closest to the obstruction will be the strongest with the waves further
downstream getting progressively weaker.
• a large eddy called a “rotor” may form below each wave crest.
• mountain ranges downstream may amplify or nullify induced wave patterns.
• downdrafts are frequently found on the downwind side of the obstruction.
These downdrafts typically reach values of 2,000 feet per minute but downdrafts up to 5,000 feet per minute have been reported. The strongest downdraft
is usually found at a height near the top of the summit and could force an aircraft into the ground.
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W
A
Fig. 2-22 - Amplitude (A) and wavelength (W) in lee waves
65 Knots
55 Knots
45 Knots
48 Knots
Fig. 2-23 - Stronger wind in wave crest in lee waves
Fig. 2-24 - A rotor may form beneath wave crests
Clouds Associated with Lee Waves
Lee waves involve lift and, if sufficient moisture is available, characteristic clouds
will form. The signature clouds may be absent, however, due to the air being too dry
or the cloud being embedded within other clouds and not visible. It is essential to
realize, nevertheless, that the absence of lee wave clouds does not mean that there are
no lee waves present.
(a) Cap cloud
A cloud often forms over the peak of the mountain range and remains stationary. Frequently, it may have an almost “waterfall” appearance on the leeward
side of the mountain. This effect is caused by subsidence and often signifies a
strong downdraft just to the lee of the mountaintop.
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(b) Lenticular clouds
A lens shaped cloud may be found at the crest of each wave. These clouds may
be separated vertically with several thousand feet between each cloud or may
form so close together they resemble a “stack of plates.” When air flows
through the crest it is often laminar, making the cloud smooth in appearance.
On occasion, when the shear results in turbulence, the lenticular cloud will take
on a ragged and wind torn appearance.
(c) Rotor cloud
A rotor cloud may form in association with the rotor. It will appear as a long
line of stratocumulus, a few miles downwind and parallel to the ridge. Its base
will be normally below the peak of the ridge, but its top can extend above it.
The turbulence associated with a rotor cloud is severe within and near the rotor
cloud.
LENTICULAR
CAP
ROTOR
Fig. 2-25 - Characteristic clouds formed by lee waves
Fronts
A front is the transition or mixing zone between two air masses. While only the
surface front is shown on a weather map, it is important to realize that an air mass is
three-dimensional and resembles an “wedge”. If the colder air mass is advancing, then
the leading edge of the transition zone is described as being a cold front. If the colder
air mass is retreating, then the trailing edge of the transition zone is described as
being a warm front.
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Fig. 2-26 - Cross-section of a cold front
The movement of a front is dependent on the motion of the cold air nearly perpendicular to the front, both at the surface and aloft. When the winds blow across a
front, it tends to move with the wind. When winds blow parallel to a front, the front
moves slowly or even becomes quasistationary. The motion of the warm air does not
affect the motion of the front.
On surface charts, fronts are usually drawn as relatively straight lines. In reality, this
is seldom so. Cold air flows across the surface like water. When advancing, it readily
moves across level ground but in hilly or mountainous terrain it is held up until it
either finds a gap or deepens to the point where it can flow over the barrier. Cold air
also readily accelerates downhill resulting in rapid motion along valleys. When
retreating, cold air moves slowly and leaves pools of cold air in low-lying areas that
take time to modify out of existence.
Frontal Weather
When two different air masses encounter each other across a front, the cooler,
denser air will lift the warm air. When this happens, the weather at a front can vary
from clear skies to widespread cloud and rain with embedded thunderstorms. The
weather occurring at a front depends on:
(a) amount of moisture available
Sufficient moisture must be present for clouds to form. Insufficient moisture
results in “dry” or “inactive” fronts that may be marked by only changes of temperature, pressure and wind. An inactive front can become active quickly if it
encounters an area of moisture.
(b) stability of the air being lifted
The degree of stability influences the type of clouds being formed. Unstable air
will produce cumuliform clouds accompanied by showery weather and more
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turbulent conditions. Stable air will produce stratiform cloud accompanied by
steady precipitation and little or no turbulence.
(c) slope of the front
A shallow frontal surface such as a warm front produces widespread cloud and
steady precipitation. Such areas are susceptible to the formation of low stratus
cloud and fog and may have an area of freezing precipitation. Passage of such a
front is usually noted by the end of the steady precipitation, followed by a slow
reduction in the cloud cover.
A steep frontal surface, such as is seen in cold fronts, tends to produce a narrow
band of convective weather. Although blustery, the period of bad weather is
short-lived and the improvement behind the front is dramatic.
(d) speed of the front
A fast-moving cold front enhances the vertical motion along the front, which,
in turn, causes the instability to be accentuated. The result is more vigorous
convective-type weather and the potential for the development of squall lines
and severe weather.
Frontal Waves and Occlusions
Small-scale changes in pressure along a front can create localized alterations in the
wind field resulting in a bending of the front. This bending takes on a wave-like
appearance as part of the front begins to move as a warm front and another part
moves as a cold front. Such a structure is known as a frontal wave. There are two types
of frontal waves:
(a) Stable Waves
The wave structure moves along the front but does not develop beyond the
wave appearance. Such features, known as stable waves, tend to move rapidly
(25 to 60 knots) along the front and are accompanied by a localized area of
heavier cloud and precipitation. The air mass stability around the wave determines the cloud and precipitation type. Since the wave moves rapidly, the associated weather duration tends to be short.
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Warm Air
▼
◗
◗
◗
▼
▼ ▼
◗
◗
Cold Air
▼ ▼
◗
▼
◗
▼
◗
◗
▼
◗
▼
◗
▼
◗
Cold Air
▼
◗
▼
◗
Warm Air
◗
◗
Cold Air
◗
▼
◗
◗
▼ ▼
Warm
Sector
Fig 2-27 - Stable wave
◗
◗
◗
◗
◗
Warm Air
◗
◗
◗
▼
▼
◗
▼ ▼ ▼ ▼ ▼
◗
▼
Wave
Crest
◗
▼
▼
Cold
Air
◗
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Warm Air
(b) Unstable (Occluding) Waves
Given additional support for development, such as an upper trough, the surface
pressure will continue to fall near the frontal wave, causing the formation of a
low pressure centre and strengthening winds. The wind behind the cold front
increases causing the cold front to accelerate and begin to wrap around the low.
Eventually, it catches up with the warm front and the two fronts occlude or
“close together.” At this point, the low is at maximum intensity.
Cold
Air
Cool
Air
Trowal
Cold
Air
Cool
Air
Warm Air
Cold
Air
Trowal
Cool
Air
Warm Air
Cold
Air
Warm Air
Fig. 2-28 - Formation of an occluding wave
Occlusions occur because the air behind the cold front is colder and denser than the
cool air mass ahead of the warm front. Thus, it undercuts not only the warm sector of
the original wave but also the warm front, forcing both features aloft. As the warm
sector is lifted higher and higher, the surface portion becomes smaller and smaller.
Along the occlusion, the weather is a combination of a warm front and a cold front;
that is, a mix of layer clouds with steady precipitation and embedded convective
clouds with enhanced showery precipitation. Such a cloud mass should be approached
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with caution as both icing and turbulence can be quite variable. Eventually, the frontal
wave and occlusion both move away from the low, leaving only an upper frontal band
curling back towards the low. This upper structure continues to weaken as it moves
further and further away from the low that initially formed it .
E
D
C
A
B
WARM AIR
COLD
AIR
WARM AIR
COOL
AIR
COLD
AIR
A
WARM AIR
COOL
AIR
COLD
AIR
D
COOL
AIR
E
F
Fig. 2-29 - Frontal cross-sections
Thunderstorms
No weather encountered by a pilot can be as violent or threatening as a thunderstorm. Thunderstorms produce many hazards to the aviation community, and since
they are so commons in summer time, it is important that pilots understand their
nature and how to deal with them. To produce a thunderstorm, there are several
ingredients which must be in place. These include:
• An unstable airmass
• Moisture in the low levels
• Something to trigger them, e.g. daytime heating, upper level cooling
• For severe thunderstorms, wind shear.
No weather encountered by a pilot can be as unpredictable or as violent as a thunderstorm. There are few severe weather hazards that a thunderstorm does not pack,
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and with thunderstorms being common, it is important that a pilot understands a
little about their nature and how to deal with them.
The Life Cycle of a Thunderstorm
The thunderstorm, which may cover an area ranging from 5 miles in diameter
to, in the extreme case, as much as 50 miles, usually consists of two or more cells in
different stages of their life cycle. The stages of life of individual cells are:
Dissipating
Cell
Mature
Cell
Mature
Cell
Cumulus
Cell
Fig. 2-30 -Top-down view of a thunderstorm "family" containing
cells in different stages of development
(a) Cumulus Stage
The cumulus stage is marked by updrafts only. These updrafts can reach values
of up to 3,000 feet per minute and cause the cloud to build rapidly upwards,
carrying supercooled water droplets well above the freezing level. Near the end
of this stage, the cloud may well have a base more than 5 miles across and a
vertical extent in excess of 20,000 feet. The average life of this stage is about 20
minutes.
(b) Mature Stage
The appearance of precipitation beneath the base of the cell and the development of the downdraft mark the transition to this stage. The downdraft is
caused by water drops which have become too heavy for the updraft to support
and now begin to fall. At the same time, the drops begin to evaporate as they
draw in dry air from the edge of the cloud, and then fall through the drier air
beneath the base of the cloud. This evaporation causes the air to cool and
become denser, resulting in a downwash of accelerating cold air. Typical downdraft speeds can reach values of 2,500 feet per minute.
Dissipating
Cell
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Cumulus
Cell
Fig. 2-31 - Cumulus stage
Fig. 2-32 - Mature stage
Fig. 2-33- Dissipating Stage
The downdraft, when it hits the ground, spreads out in all directions but travels
fastest in the direction that the storm is moving. The leading edge of this cold
air is called the “gust front” and can extend ten to fifteen miles, or even further,
when channelled along mountain valleys in front of the storm. A rapid drop in
temperature and a sharp rise in pressure characterize this horizontal flow of
gusty surface winds.
At the same time, the updrafts continue to strengthen until they reach maximum speeds, possibly exceeding 6,000 feet per minute. The cloud reaches the
tropopause which blocks the updraft, forcing the stream of air to spread out horizontally. Strong upper winds at the tropopause level assist in the spreading out
of this flow in the downwind direction, producing the traditional anvil-shaped
top. This is classically what is referred to as a cumulonimbus cloud (CB).
The thunderstorm may have a base measuring from 5 miles to more than
15 miles in diameter and a top ranging from as low as 20,000 feet to more than
50,000 feet. The mature stage is the most violent stage in the life a thunderstorm
and usually lasts for 20 to 30 minutes.
Near the end of the mature stage, the downdraft has increased in size so that
the updraft is almost completely “choked off,” stopping the development of the
cell. However, at times, the upper winds increase strongly with height causing
the cell to tilt. In such a case, the precipitation falls through only a portion of
the cell, allowing the updraft to persist and reach values of 10,000 feet per
minute. Such cells are referred to as “steady state storms” that can last for several hours and produce the most severe weather, including tornadoes.
(c) Dissipating Stage
The dissipating stage of a cell is marked by the presence of downdrafts only.
With no additional flow of moisture into the cloud from an updraft, the rain
gradually tapers off and the downdrafts weaken. The cell may dissipate completely in 15 to 30 minutes, leaving clear skies or patchy cloud layers. At this
stage the anvil, which is formed almost exclusively of ice crystals, often detaches and drifts off downwind.
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Types of Thunderstorms
(a) Air Mass Thunderstorms
These thunderstorms form within a warm, moist air mass and are non-frontal
in nature. They are usually a product of diurnal heating, tend to be isolated,
reach maximum strength in the late afternoon, are seldom violent, and usually
dissipate quickly after the setting of the sun. There is also a second form of air
mass thunderstorm that is created by cold advection. In this case, cold air
moves across warm land or water and becomes unstable. Of these two, it is
the movement of cold air over warm water that results in the most frequent
occurrence of this type of thunderstorm. Since the heating is constant, these
thunderstorms can form at any time of day or night.
Fig. 2-34 - Air heated by warm
land
Fig. 2-35 - Cool air heated by
warm water
(b) Frontal Thunderstorms
These thunderstorms form either as the result of a frontal surface lifting an
unstable air mass or a stable air mass becoming unstable, as a result of the lifting. Frontal thunderstorms can be found along cold fronts, warm fronts and
trowals. These thunderstorms tend to be numerous in the area, often form in
lines, are frequently embedded in other cloud layers, and tend to be active during the afternoon and well into the evening. Cold frontal thunderstorms are
normally more severe than warm frontal thunderstorms.
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CB
CB
CI
CS
NS
AC/AS
ST
Fig. 2-36 - Warm frontal thunderstorms
(c) Squall Line Thunderstorms
A squall line (or line squall) is a line of thunderstorms. Squall lines can be several hundred miles long and have lower bases and higher tops than the average
thunderstorm. Violent combinations of strong winds, hail, rain and lightning
makes them an extreme hazard not only to aircraft in the air, but also to those
parked uncovered on the ground.
Squall line thunderstorms are most often found 50 to 300 miles ahead of a
fast-moving cold front but can also be found in accompanying low pressure
troughs, in areas of convergence, along mountain ranges and even along sea
breeze fronts.
(d) Orographic Thunderstorms
Orographic thunderstorms occur when moist, unstable air is forced up a mountain slope. These are common along the slopes of the Rocky Mountains where,
on a typical summer day, they form due to daytime heating and then move
eastward in the upper flow. If conditions are favourable they can persist for several hours otherwise, they dissipate fairly rapidly. Typically they will begin to
develop near noon and can continue to form well into the afternoon. In such
situations, these storms frequently produce copious amounts of hail.
CB
TCU
Fig. 2-37 - Orographic thunderstorms
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(e) Nocturnal Thunderstorms
Nocturnal thunderstorms are those that develop during or persist all night.
Usually, they are associated with an upper level weather feature moving through
the area, are generally isolated, and tend to produce considerable lightning.
Severe Thunderstorms
The discussion of the life cycle of a thunderstorm does not fit the case of those that
seem to last for extended periods of time and are most prolific in producing tornadoes and large hail. A particular type of severe thunderstorm is known as a
“Supercell”.
A severe storm typically begins as a multi-cellular thunderstorm. However, because
the upper winds increase strongly with height, the cell begins to tilt. This causes the
descending precipitation to fall through only a portion of the cell, allowing the
updraft to persist.
The second stage of the life cycle is clearly defined by the weather. At this stage,
the largest hail fall generally occurs and funnel clouds are often observed.
The third and final stage of storm’s evolution is the collapse phase. The cell’s downdrafts increase in magnitude, and extend horizontally, while the updrafts are decreasing. It is at this time that the strongest tornadoes and straight-line winds occur.
Fortunately these types of storms are rare in BC and are almost exclusivily confined
to the central and northern part of the province.
Thunderstorm Hazards
The environment in and around a thunderstorm can be the most hazardous
encountered by an aircraft. In addition to the usual risks such as severe turbulence,
severe clear icing, large hail, heavy precipitation, low visibility and electrical discharges within and near the cell, there are other hazards that occur in the surrounding environment.
(a) The Gust Front
The gust front is the leading edge of any downburst and can run many miles
ahead of the storm. This may occur under relatively clear skies and, hence, can
be particularly nasty for the unwary pilot. Aircraft taking off, landing, or operating at low levels can find themselves in rapidly changing wind fields that
quickly threaten the aircraft’s ability to remain airborne. In a matter of seconds,
the wind direction can change by as much 180°, while at the same time the
wind speed can approach 100 knots in the gusts. Extremely strong gust fronts
can do considerable damage on the ground and are sometimes referred to as
“plow winds.” All of this will likely be accompanied by considerable mechanical
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turbulence and induced shear on the frontal boundary up to 6,500 feet above
the ground.
(b) Downburst, Macroburst and Microburst
A downburst is a concentrated, severe downdraft which accompanies a
descending column of precipitation underneath the cell. When it hits the
ground, it induces an outward, horizontal burst of damaging winds. There are
two types of downburst, the “macroburst” and the “microburst”.
A macroburst is a downdraft of air with an outflow diameter of 2.2 nautical
miles, or greater, with damaging winds that last from 5 to 20 minutes. Such
occurrences are common in the summer but only rarely hit towns or airports.
On occasion, embedded within the downburst, is a violent column of descending air known as a “microburst”. Microbursts have an outflow diameter of less
than 2.2 nautical miles and peak winds lasting from 2 to 5 minutes. Such winds
can literally force an aircraft into the ground.
Cloud
External Downdraft
due to evaporation
cooling
Direction of travel
Inflow to storm
Cold air
Outflow from storm
Fig. 2-38 - “Steady state” tilted thunderstorm
Gust front
Fig. 2-39 - The gust front
(c) Funnel Cloud, Tornado and Waterspout
The most violent thunderstorms draw air into their base with great vigor. The
incoming air tends to have some rotating motion and, if it should become concentrated in a small area, forms a rotating vortex in the cloud base in which
wind speeds can exceed 200 knots. If the vortex becomes strong enough, it will
begin to extend a funnel-shaped cloud downwards from the base. If the cloud
does not reach the ground, it is called a funnel cloud. If it reaches the ground,
it is refered to as a tornado and if it touches water, it is a waterspout.
Any severe thunderstorm should be avoided be a wide margin as all are extremely
hazardous to aircraft.
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Photo 2-3 - Severe thunderstorm
F-Scale
Number
Intensity Phrase
Wind Speed
(kts)
credit Alister Ling
Type of Damage Done
FO
Weak
Tornado
35-62
Some damage to chimneys; breaks branches off trees;
pushes over shallow-rooted trees; damages sign boards.
F1
Moderate
Tornado
63-97
The lower limit is the beginning of hurricane wind speed;
peels surface off roofs; mobile homes pushed off foundations
or overturned; moving autos pushed off the roads; attached
garages may be destroyed.
F2
Strong
Tornado
98-136
Considerable damage. Roofs torn off frame houses;
mobile homes demolished; boxcars pushed over; large trees
snapped or uprooted; light-object missiles generated.
F3
Severe
Tornado
137-179
Roof and some walls torn off well constructed houses;
trains overturned; most trees in forest uprooted
F4
Devastating
Tornado
180-226
Well-constructed houses leveled; structures with weak
foundations blown off some distance; cars thrown and
large-object missiles generated.
F5
Incredible
Tornado
227-285
Table 2-1 - The Fujita Scale
Strong frame houses lifted off foundations and carried
considerable distances to disintegrate; automobile-sized
missiles fly through the air in excess of 100 meters; trees
debarked; steel re-inforced concrete structures badly damaged.
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Photo 2-4 - Waterspout off the west coast of Vancouver Island
credit: Canadian
Armed Forces
Waterspouts are most often observed over coastal BC when cold air is moving
across warm water. The first sign that a waterspout may form is the cloud sagging
down in one area. If this bulge continues downward to the sea surface, forming a
vortex beneath it, water will be carried aloft in the lower 60 to 100 feet.
Cold Weather Operations
Operating an aircraft in extremely cold weather conditions can bring on a unique
set of potential problems.
Temperature Inversion and Cold Air Outbreaks
Low level inversions are common in most areas during the fall and winter due to
very cold outbreaks and strong radiation cooling. When cold air moves out over the
open water, it becomes very unstable. Cloud can be seen to almost be “boiling” off the
waters surface and vortices of cloud have been witnessed to rotate upwards off the
water into the cloud. Such a condition can be very turbulent and there is a significant
risk of serious icing. At the same time, the convection enhances any snowfall resulting in areas of extremely poor visibility.
Looming
Another interesting effect in cold air is the bending of low angle light rays as they
pass through an inversion. This bending creates an effect known as “looming,” a form
of mirage that causes objects normally beyond the horizon to appear above the horizon.
Ice Fog and Ice Crystals
Ice fog occurs when water vapour sublimates directly to ice crystals. In conditions
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of light winds and temperatures colder than -30°C or so, such as those that might be
found in Fort Nelson, water vapour from anthropogenic sources (man-made) can
form widespread and persistent ice fog or ice crystals. In light winds, the visibility can
be reduced to near zero, closing an airport for hours.
Blowing Snow
Blowing snow can occur almost anywhere where dry snow can be picked up by
strong winds but poses the greatest risk away from the forested areas of the Prairies.
As winds increase, blowing snow can, in extreme conditions, reduce horizontal visibility at runway level to less than 100 feet.
Whiteout
“Whiteout” is a phenomena that can occurs when a layer of cloud of uniform
thickness overlays a snow or ice-covered surface, such as a large frozen lake. Light rays
are diffused when they pass through the cloud layer so that they strike the surface
from all angles. This light is then reflected back and forth between the surface and
cloud, eliminating all shadows. The result is a loss of depth perception, the horizon
becoming impossible to discern, and dark objects seeming to float in a field of white.
Disastrous accidents have occurred under such conditions where pilots have flown
into the surface, unaware that they were descending and confident that they could see
the ground.
Altimetry Errors
The basic barometric altimeter in an aircraft assumes a standard change of temperature with height in the atmosphere and, using this fact, certain pressure readings by
the altimeter have been defined as being at certain altitudes. For example, a barometric altimeter set at 30.00" would indicate an altitude of 10,000 feet ASL when it senses the outside pressure of 20.00".
Cold air is much more dense than the assumed value used in the standard ICAO
atmosphere. For this reason, any aircraft that is flying along a constant pressure surface will actually be descending as it moves into areas of colder air, although the indicated altitude will remain unchanged. Interestingly enough, a new altimeter setting
obtained from a site in the cold air will not necessarily correct this problem and may
increase the error.
Consider:
A pilot obtained an altimeter setting of 29.85" and plans to maintain a flight level
of 10,000 feet enroute. As the aircraft moves into an area with a strong low-level
inversion and very cold surface temperatures, the plane descends gradually as it
follows the constant pressure surface corresponding to an indicated altitude of 10,000
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feet. A new altimeter setting, say 30.85 inches, is obtained from an airport located in
the bottom of a valley, deep in the cold air. This new setting is higher than the
original setting and, when it is entered, the altimeter will show an increase in altitude
(in this case the change is one inch and so the altimeter will show an increase from
10,000 to 11,000 feet). Unaware of what is happening, the pilot descends even
further to reach the desired enroute altitude, compounding the height error.
If the aircraft were operating in cloud-shrouded mountains, an extremely hazardous situation can develop. There is no simple solution to this problem, other than
to be aware of it and allow for additional altitude to clear obstacles.
Volcanic Ash
A major, but fortunately infrequent, threat to aviation is volcanic ash. When a
volcano erupts, a large amount of rock is pulverized into dust and blasted upwards.
The altitude is determined by the severity of the blast and, at times, the ash plume
will extend into the stratosphere. This ash is then spread downwind by the winds aloft
in the troposphere and the stratosphere.
The dust in the troposphere settles fairly rapidly and can limit visibility over a large
area. For example, when Mt. St. Helens, Washington, erupted, there was ash fallout
and limited visibility across Washington State, just to the south of the borders.
Of greater concern is the volcanic ash that is ingested by aircraft engines at flight
level. Piston-driven engines have failed due to plugged air filters while turbine
engines have “flamed out.”
The volcanic dust also contains considerable pumice material. Leading edges such
as wings, struts, and turbine blades can all be abraded to the point where replacement
becomes necessary. Windscreens have been abraded until they become opaque.
Deformation Zone
A deformation zone is defined as “an area in the atmosphere where winds converge
along one axis and diverge along another. Deformation zones (or axis of deformation
as they are sometimes referred to) can produce clouds and precipitation.” More simply put, we are referring to areas in the atmosphere where the winds flow together
(converge) or apart (diverge), resulting in areas where air parcels undergo stretching
along one axis and contraction along another axis. Meteorologically, this is an area
where significant cloud amounts, precipitation, icing and turbulence can occur to in
the induced vertical currents.
For meteorologists, the most common form of deformation zones are the ones
associated with upper lows. Northeast of the upper low, a deformation zone usually
forms in which the air is ascending. In this area, thick cloud layers form giving wide-
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spread precipitation. Depending on the temperatures aloft, this cloud may also contain significant icing. During the summer, the edges of this cloud area will often have
thunderstorms develop in the afternoon. If this area of cloud is slow moving, or
should it interact with terrain, then the upslope areas can see prolonged precipitation.
Winds shear in the ascending air will often give turbulence in the middle-and
higher-levels.
A second deformation zone exists to the west and northwest of these lows. In this
case the air is descending, so that widespread higher clouds usually only consist of
whatever cloud is wrapped around the low. Precipitation here tends to be more intermittent or showery. Wind shear can also cause turbulence but most often it is confined to the low-levels.
L
H
Fig. 2-40 - Deformation zones
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Notes:
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Chapter 3
Weather Patterns of British Columbia
Introduction
“Weather is what you get, climate is what you expect.”
Weather is what happens. Weather is also transitory, seldom lasting more than a
matter of hours. Climate speaks to the weather history of a location, the how and why
the weather varies between seemingly identical locations. Why is Abbotsford open
when all other nearby airfields are closed in fog? What are the predominate winds at
Penticton? Meteorologists use their knowledge of both weather and climate when
producing forecasts. This constant conflict between “what you expect” and “what you
get” is unending; it is a problem that becomes much more difficult when you have to
take mountainous terrain into consideration.
Geography of British Columbia
Northeast BC
Central & Northern
Interior
North
Coast
South
Coast
Map 3-1 - Topography of GFA 31 Domain
ThompsonOkanagan
Kootenays
&
Columbias
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CHAPTER THREE
At first glimpse, British Columbia can quickly be divided into two distinctive areas;
the coast and the interior. The coast of British Columbia, almost 500 miles in length,
lies at mid-latitudes along the western boundary of North America. To the west of it
lies the largest ocean in the world, the Pacific Ocean.
The ocean edge is itself dominated by the Coast Mountains that seem to rise right
out of the sea. This range of mountains contains numerous valleys, some of which are
flooded with water from the ocean resulting in a string of islands and coastal inlets.
On the leeward side of the Coastal Mountains lie the interior regions of British
Columbia, a mixture of mountain ranges, deep valleys and plateau areas. The most
prominent feature of the interior is the Rocky Mountains. These mountains extend
out of the United States along the Alberta - British Columbia border to near Jasper,
then continue northwestward to pass just to the west of Fort Nelson.
To the east of the northern Rocky Mountains lies Northeastern British Columbia.
An extension to the Canadian Prairies, the terrain here is almost flat, rising steadily
in elevation from the Alberta border until it reaches the Rocky Mountains.
South Coast
10,000 FT
7000 FT
5000 FT
3000 FT
2000 FT
PORT HARDY
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
CAMPBELL RIVER
COMOX
PEMBERTON
TOFINO
VANCOUVER
NANAIMO
ABBOTSFORD
VICTORIA
Map 3-2 - The South Coast
HOPE
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The South Coast runs for approximately 250 nautical miles from the
Canada/United States border to just north of Vancouver Island. The general flow
pattern is dominated by the westerlies which means that moist weather systems are
spawned over the Pacific Ocean and carried into the coast.
The ocean also supplies heat which acts to moderate the temperatures along the
coast, as compared to the inland areas. In turn, this moderation of temperature plays
a significant role in determining just what type of precipitation will fall over the area.
At the same time, the interaction of the warm air with the cold air “energizes” lows.
During the winter, a typical low approaching the coast will begin to draw cold air
down from the north. This causes the low to intensify resulting in rapidly falling pressures and strengthening winds.
Dividing the moist coastal environment from the dry environment of the interior
are the Coast Mountains, which lie along the mainland coast and the Insular
Mountains on Vancouver Island. These mountains have an average height of 6,000 to
7,000 feet ASL; however, just to the east of Campbell River the Coast Mountains rise
to an average height of 10,000 feet. Along this barrier of rugged, tree covered slopes
are numerous valleys, some dry, some flooded, of which the Fraser Valley is the largest
and most significant.
Mountains impact significantly on the weather in this area. Storms approaching
the coast are lifted rapidly along the windward slopes, resulting in widespread
precipitation. Much of the heaviest precipitation occurs along the western slopes of
the Insular Mountains and the Olympic Mountains of Northern Washington State
(elevation 6,000 to 8,000 feet). The variability in annual precipitation between
upslope areas and subsident “rain shadow” areas is significant. Tofino, on the west
coast of Vancouver Island, receives an average of 3,000 millimetres of precipitation
each year while Nanaimo, on the east coast of Vancouver Island, receives a paltry
1,150 millimetres. The Coast Mountains also serve as a barrier to the arctic air that
occasionally moves into the interior so that only the strongest incursion can force its
way through the various passes and valleys.
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CHAPTER THREE
North Coast
MASSET
PRINCE RUPERT
TERRACE
SANDSPIT
10,000 FT
7000 FT
5000 FT
3000 FT
2000 FT
BELLA BELLA
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
Map 3-3 - The North Coast
The North Coast is approximately 300 miles in length and extends in a northnorthwest to south-southeast line from just north of Vancouver Island to Stewart. To
the west of these mountains lie the Queen Charlotte Islands and the Pacific Ocean.
In many ways the North Coast can be considered as a “nastier” version of the South
Coast. Lacking the sheltering bulk of Vancouver Island, ocean storms can run right
up onto the coast venting their full fury.
Along the coast runs the northern extension of the Coast Mountains, with a mean
height of 6,500 feet ASL. These mountains are carved with numerous flooded
valleys, resulting in a string of islands and coastal inlets. Sixty miles off the coast
are the Queen Charlotte Islands whose Insular Mountains rise to an average height
of 3,000 feet.
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The heat from the ocean strongly moderates coastal temperatures; however, outflow
winds through the inland valleys can easily carry cold air from the interior. During
the winter this clash between warmer coastal air and cold air flowing out of the interior can make for extremely variable precipitation types.
Thompson-Okanagan
KAMLOOPS
LYTTON
10,000 FT
7000 FT
5000 FT
VERNON
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
KELOWNA
300 FT
0 SEA LEVEL
PENTICTON
Map 3-4 - Thompson-Okanagan
The Thompson-Okanagan extends from the Fraser Canyon in the west to the
slopes of the Monashee Mountains in the east, and from the US/Canada border
to and east-west line just to the north of Kamloops and Shuswap Lake. The
topography consists of a mixture of mountains and valleys. The mountains are largely
tree-covered with extensive cut blocks (areas that have been logged) that are in various stages of re-growth. The valley bottoms are dry except for rivers and lakes, and
contain some of the largest population centres in the interior.
The weather in this area tends to be benign for the large part and is controlled to
a great extend by the Coast Mountains. Sitting to the lee of these mountains, subsidence has reduced the precipitation so that much of the area is arid or semi-arid.
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CHAPTER THREE
Annual precipitation ranges between 250 and 360 millimetres per year and tends to
be divided up nearly equally between the various months.
Southwesterly
Flow
Broken clouds
Clouds
Clouds
Pacific Ocean
Showers
Rain
Rain
Vancouver
Island
Mountains
Coast Mountains
Vancouver
Monashees
Kelowna
Fig. 3-1 - The most notable effect of mountains is their impact on precipitation
Summer in this area is noted for the incursion of the Pacific High and the
development of hot and dry weather. The weather tends to be dry and sunny with late
afternoon or evening thunderstorms occurring mainly along the ridges. Eventually,
the Pacific High does break down and, when this happens, it is common for
widespread thunderstorms to develop as cooler, moist air begins to move into the area.
Winters are a different story. Mountain valleys allow cold air to pool, creating
inversions. Most of the valleys have rivers and lakes that seldom freeze up resulting in
abundant moisture that the inversion can trap, supporting the development of low
“valley cloud”. On the positive side, because of its location, only the strongest incursions of arctic air can force its way into this area. This being said, the temperatures in
the area do tend to hover around freezing, and the cold surface layer can be difficult
to remove, as the warm air moving in from the coast rides over the top of the cold air.
The only real warming occurs with southerly winds, but this respite only lasts a few
days as cold air is quick to re-establish itself in the valley bottoms.
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Kootenays and Columbias
VALEMOUNT
10,000 FT
GOLDEN
7000 FT
5000 FT
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
FAIRMONT
CASTLEGAR
CRANBROOK
Map 3-5 - The Kootenays and Columbias
The eastern and southeastern section of the interior consists of a mixture of mountain ranges and deep valleys. The mountains are largely tree-covered and the valleys
narrow and steeply-sided. The valley bottoms contain rivers and lakes with little in
the way of cleared areas other than around towns and forestry cuts.
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CHAPTER THREE
The most prominent feature in this area is the Rocky Mountains. These mountains
extend out of the United States along the Alberta - British Columbia border to Jasper
then continue on in a northwesterly direction into the Yukon. Over the southern half
of the province, the Rocky Mountains rise to average of 9,000 to 11,000 feet ASL.
To the west of the Rockies, the area is carved up by a series of other mountain
ranges: the Monashees, the Selkirks, and the Purcells, with narrow valleys between
them. Most of these valleys contain rivers or lakes, along with the population centres
of various sizes.
A prominent feature of British Columbia is the Rocky Mountain Trench. A broad
gash in the terrain, the Trench begins in the south near Cranbrook and moves northwestward through Golden and Valemount, passes just to the east of Prince George,
then continues north-northwestward.
The pronounced variation in terrain in the Interior has a strong influence on its
climate. On the small scale, the changing topographical features such as elevation,
orientation to the mean winds, slope, and exposure all combine to alter the local
climate. Higher elevations tend to have lower average temperatures and increased
precipitation. Low-lying areas, such as valleys, tend to allow cold air to drain into
them, creating higher occurrences of frost and fog.
On the larger scale, the Rocky Mountains act as a barrier. All but the strongest
incursions of Arctic air are held out of the southern British Columbia interior, maintaining temperatures much above those on the Prairies.
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Central and Northern Interior
10,000 FT
7000 FT
5000 FT
3000 FT
2000 FT
1500 FT
1000 FT
WATSON LAKE
600 FT
300 FT
0 SEA LEVEL
DEASE LAKE
WARE
INGENIKA
SMITHERS
MACKENZIE
PRINCE GEORGE
PUNTZI MOUNTAIN
WILLAMS LAKE
Map 3-6 - The Central and Northern Interior
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CHAPTER THREE
The Central and Northern Interior, like the rest of BC, are largely mountainous;
however, there are two major plateaux located in this area. These are the Central
Interior Plateau, west of Williams Lake and Prince George, and the Atlin-Stikine
Plateau, over northwestern British Columbia. Throughout the area are a series of
rivers and lakes, some of which, such as Quesnel and Williston Lakes, are quite large.
The Rocky Mountain Trench is also prominent in this area. Beginning in the
southeast interior, it passes just to the east of Prince George, forms Williston Lake,
then narrows and continues northwestward towards Watson Lake in the Yukon.
The plateaux have a significant impact on the weather. During the summer, the
flattened terrain can develop significant convection. Moderate to heavy thunderstorms are not uncommon and, on occasion, a tornado or funnel cloud will be reported in the Prince George area. Winter produces its own problems. The incursion of
arctic air is fairly common and, this, coupled with moisture from local forestry mills,
produces widespread and frequent low stratus and fog. Even when warm air does
invade from the Pacific, it frequently flows over top of the cold air at low levels, making it difficult to erode the stratus and fog.
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Northeast British Columbia
FORT NELSON
FORT ST JOHN
10,000 FT
7000 FT
5000 FT
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
Map 3-7 - Northeast British Columbia
The western boundary of this area is marked by the Rocky Mountains that lie in a
north-northwest to south-southeast line from Jasper to just west of Fort Nelson and
then into the Yukon. While not as high as the southern part of the Rockies, they still
rise to a respectable average height of 7,000 feet ASL. To the east of the Rockies is
an extension to the Canadian Prairies; the terrain here is almost flat, rising steadily in
elevation from the Alberta border until it reaches the Rocky Mountains.
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CHAPTER THREE
The climate here is more like that of Alberta than the remainder of British
Columbia. The summer tends to be hot and convective. The occasional cold low does
move across the area; however, giving widespread precipitation and cloudy, cool
conditions. Winter is cold as arctic air dominates. Warm air moving eastward overtop
the cold air will give snow but does little to moderate the temperature. Despite this,
a strong southwesterly flow will produce Chinook conditions that drive temperatures,
in such places as Fort St. John, above freezing.
Mean Upper Atmospheric Circulation
L
H
L
Aleutian
Low
L
L
Aleutian
Low
H
Pacific
High
H
Pacific
High
Fig. 3-2 - Typical winter pattern
Fig. 3-3 - Typical summer pattern
The mean circulation of the upper level winds over British Columbia is the result
of two semi-permanent features. The “Pacific High” extends from the area east of
Hawaii to the West Coast of the United States. The “Aleutian Low” occurs in the
Gulf of Alaska and over the Aleutian Islands. The combination of the counterclockwise circulation around the Aleutian Low and the clockwise circulation around the
Pacific High results in a mean westerly flow onto the west coast of North America.
The Aleutian Low is strongest in the winter, which causes the mean circulation to
take on a southwesterly component, while the Pacific High is strongest in the
summer causing a more northwesterly flow. Complicating this simple relationship
are the upper troughs of low pressure, upper ridges of high pressure and other weather
systems that move along with the upper winds, altering or disrupting the normal
pattern.
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Upper Troughs and Upper Ridges
UPPER RIDGE
UPPER TROUGH
H
Fig.3-4 - Typical winter pattern with upper troughs and ridges
added
Although the mean upper flow is generally from west to east, there tends to be a
series of upper troughs and ridges embedded within this flow. Upper troughs induce
vertical lift in the atmosphere, which in turn is associated with cloud and precipitation. In the winter, these troughs are at their strongest and frequently create broad
areas of cloud with widespread precipitation. This process can be further enhanced if
an orographic feature lifts the air mass at the same time. During the summer months,
the cloud shields associated with upper troughs are narrower and usually quite
convective in nature. The vertical lift and upper level cooling produced by the trough
will intensify any preexisting instability in the atmosphere, and some of the worst
summertime thunderstorms are produced when such a trough moves over a region
that has already been destabilized by daytime heating. If an upper trough crosses an
area where strong baroclinicity (temperature gradient) exists, it can set off a chain of
events that results in the development of a surface low pressure system and/or frontal
wave. These will further enhance the cloud and precipitation. Clearing behind an
upper trough can be gradual in the winter, but tends to be quite rapid in the summer.
Any surface pressure system associated with an upper trough moving eastward
across British Columbia often fills (weakens) as the system encounters the mountainous terrain.
Upper ridges, on the other hand, are associated with clear skies and good weather
as they induce regions of descent within the atmosphere. It is common, both in the
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summer and winter, to have a large north-south upper ridge sitting over British
Columbia. This pattern is frequently stationary for days in a row.
Both the upper trough and the upper ridge interact with the Pacific High and the
Aleutian Low. An upper ridge will strengthen the Pacific High and weaken the
Aleutian Low. This tends to dissipate or divert cloud northwards away from the
British Columbia coast and into Alaska, or across the Yukon. An upper trough will
cause the Aleutian Low to deepen and the Pacific High to weaken, so that weather
systems will follow a general westerly track over the Gulf of Alaska and into British
Columbia.
Semi-Permanent Surface Features:
1016
1014
1012
1010 10081006 1004
1004
H
1002
1002
L
1004
1006
1000
ICELANDIC
LOW
L
ALEUTIAN
LOW
1008
1010
1012
1014
1016
H
PACIFIC
HIGH
H
BASIN
HIGH
1016
BERMUDA
HIGH
1016
Fig. 3-5 - January mean sea level pressure
1008
1010
L
1012
H
1014
1016
1008
1018
L
1020
H
PACIFIC
HIGH
1020
1018
L
THERMAL
1016
1014
1010
1006
1006
1010
1014
Fig. 3-6 - July mean sea level pressure
BERMUDA
HIGH
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There is a tendency for the flow of the atmosphere to conform to certain recurring
patterns, partly due to the physical geography of the earth below. In particular, there
are four large scale and semi-permanent pressure systems affecting Canadian weather.
These features are named for the regions where they normally occur:
1. The Icelandic Low located over the north Atlantic near Iceland.
2. The Aleutian Low located south or southwest of Alaska.
3. The Subtropical High located in the Atlantic near the island of Bermuda.
4. The Pacific High located off the west coast of the United States, sometimes
extending inland in the winter forming the so-called “Basin High”.
The January mean sea level surface pressure chart shows that the Aleutian low is
well out in the Pacific and the Icelandic low is southeast of Greenland. A ridge of
high pressure extends from the southwest U.S.A. northwards across Alberta into the
Mackenzie River Valley. As the year advances towards summer, both the Aleutian
and Icelandic Lows weaken. The July mean sea level surface pressure chart shows that
the Pacific High becomes established off the west coast of North America. At the
same time, a thermal low forms over the American southwest and extends a trough
into the Pacific Northwest. These influences act together to put BC under a weaker,
less predictable flow during the summer months.
Migratory Weather Systems
Closer to the surface are the migratory or traveling surface weather systems (low
pressure areas, high pressure areas, frontal systems) that are carried into British
Columbia and produce the day-to-day weather. These travelling surface weather
systems vary in intensity with the seasons and are more frequent in the winter months
(Oct. to Apr.). On average, 10 to 15 such storms will occur monthly during the winter.
Winter Storms
During the winter, low-pressure systems develop over the Pacific Ocean and move
toward the coast. Most of these storms are either Gulf of Alaska lows, which tend to
remain well offshore, or coastal lows which approach the coast before developing
rapidly. Coastal lows, while not as powerful as the Gulf of Alaska low, can be quite
dramatic in terms of their development and speed of movement.
On a few occasions during the winter, a low will combine the characteristics of both
the Gulf of Alaska low and the coastal low. In this situation, the low first moves into
the Gulf of Alaska and begins to weaken. The front sweeps over the coast giving
strong winds and widespread precipitation, especially along windward slopes. After a
time the low, which now sits over the Gulf of Alaska, begins to drift southeastward
toward the British Columbia coast and strengthens once again.
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The map shows the average tracks that the different types of lows usually follow.
The actual track of any individual storm can vary somewhat from those shown.
Gulf of
Alaska Lows
L
Coastal
Lows
H
Fig. 3-7 - Principal winter storm tracks are superimposed on a January
mean sea-level pressure pattern
Gulf of Alaska Lows
Fig. 3-8 - This sea-level pressure pattern indicates a Gulf of Alaska low
with the associatied frontal system approaching the B.C. coast
A Gulf of Alaska low usually forms south of the Aleutians as a frontal wave
between the cold northern air and warmer air to the south. Such a wave may travel a
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considerable distance eastward before it begins to take shape as a low pressure system.
Once the low starts to develop, pressures fall rapidly and the entire low pressure
system increases in size.
The low, travelling eastwards at typically 35 to 40 knots, reaches its lowest central
pressure (970 hPA or lower) over the Gulf of Alaska then turns northeastward toward
the northern end of the Alaskan Panhandle. The frontal system that accompanies the
low continues eastward onto the coast, bringing widespread cloud, precipitation, and
strong winds. Behind the cold front, a period of strong northwesterly winds of 35 to
50 knots heralds the arrival of a colder, unstable airmass.
Coastal Lows
L
Fig. 3-9 - A typical sea-level pressure pattern for a coastal low and
associated frontal system with wind pattern superimposed
Coastal lows usually intensify very rapidly just before they move over the British
Columbia coastal waters and can change from a very weak system into a severe storm
in as little as 9 hours. Lows which do develop in such a rapid, or explosive manner,
are referred to by the forecasters as “bombs.” On the coast, very strong winds will
occur usually to the east and southeast of the low, just ahead of the associated frontal
system. Winds here may reach southeasterly 70 knots with gusts to 100 knots in the
most severe storms. Often, a second band of strong winds occur behind the cold front
in the area to the southwest of the low pressure centre. Here, winds may range up to
65 knots from the west or northwest. Once the low moves ashore, it will fill rapidly
over the Coast Mountains and frequently dissipate before penetrating very far into
the interior. On occasion, the low will be able to draw down cold air from the north
and maintain its intensity. This results in a strong wind event for the interior.
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The coastal lows often move through Queen Charlotte Sound or over the Queen
Charlotte Islands. Occasionally, a low will move eastward, passing just south of
Vancouver Island. The lows which follow this southern track can bring the strongest
winds and heavy rains to the Vancouver and Victoria areas.
Winter Frontal Systems
L
Fig. 3-10 - A typical winter pressure pattern shows a front crossing the
coast with an indication of the winds near the front
While low pressure systems either die along the Coastal Mountains, or curl northward into the Gulf of Alaska, the associated frontal systems will push across the coast
and inland. The favourite track takes the frontal wave across central British
Columbia, with the trailing cold front sliding southward across the South Coast and
Southern Interior of British Columbia. An occluded front usually extends northward
from the frontal wave and moves across northern British Columbia.
As the front moves inland, it tends to weaken due to subsidence to the lee of the
Coastal Mountains but still gives steady or intermittent precipitation that will vary
with the local temperatures
Winter High Pressure Systems
Surface high pressure systems are stronger in the winter than in the summer. Along
the coast, it is the ridges of high pressure that provide the only break between active
weather systems. As the ridge of high pressure approaches the coast, the higher level
clouds dissipate and the lower cloud layer break-up, leaving scattered to broken
cumulus cloud as the developing surface ridge will cap the deep convection.
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In the Southern Interior, areas of high pressure have a lesser impact because of the
widespread valley cloud. Cold air stagnating in the bottom of the valleys causes
a strong low level inversion to form, which traps any moisture from local sources.
Under this inversion, valley cloud will form and show a great reluctance to clear.
Higher elevations; however, will be clear and cold.
The northern half of British Columbia is subject to valley cloud only during the
early part of the season, as the lakes and rivers generally freeze over completely. Thus,
ridges of high pressure during mid and late winter bring widespread clear, cold weather.
Arctic Outbreaks
❅
❅
❅
❅
❅ ❅
❅
❅
❅
❅ ❅
Fig. 3-11 - A ridge of high pressure builds over the province as the cold,
arctic air flows into the interior. This pushes the cold front out
onto the coast. Strong outflow winds occur through the mainland
inlets and near the mouth of the inlets.
During winter, a strong area of high pressure forms in the very cold air over Alaska,
the Yukon and the northern end of the Mackenzie River Valley. This cold arctic air
moves southeastwards into the Prairies but can also spread over northern and central
British Columbia. Most often, the arctic air pushes southward into the Central
Interior before coming to rest. At the same time, arctic air also flows through the
mountain passes from Alberta and fills the Rocky Mountain Trench. At least once or
twice each year, the advance of arctic air is so strong that it spreads into the Southern
Interior.
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CHAPTER THREE
Outflow
If the cold air deepens sufficiently over the interior of British Columbia, it can flow
through the coastal mountain passes down the coastal inlets, and cascade out over the
coastal waters far enough to cover the Queen Charlottes and Vancouver Island. This
condition of cold air spilling through the coastal inlets is referred to as “outflow” and
it can persist for days. Outflow is a common occurrence over the North and Central
Coast areas of British Columbia and infrequent over the South Coast.
❅
❅
❅
❅
❅
❅
❅ ❅
❅
❅
❅
❅
❅
❅
❅
❅
Fig. 3-12 - Strong winds funnel down the main- Fig. 3-13 - The end of an arctic outbreak
land inlets often reaching speeds up to 60 knots occurs when the cold air is forced back
inland by the arrival of warmer air being
driven ahead of a Pacific storm
Inflow
The end of an arctic outbreak occurs when the cold air is forced back inland by the
arrival of warmer air being driven ahead of a Pacific storm. As the low approaches
pressure will begin to fall over the interior. In response to this pressure fall, the outflow winds will gradually ease as the southeast winds strengthen along the coast.
Eventually, the pressure falls will induce an inflow of warm air. During this transition
time, from cold outflow to warm inflow, precipitation forecasting becomes very difficult. Snow changes to rain but not without the chance of freezing rain.
When a frontal system moves across the coast the rising pressures over the offshore
waters induce an inflow wind through the coastal inlets. This inflow carries the postfrontal low cloud into the inlets effectively plugging them in a manner similar to
marine stratus. This cloud will dissipate as drying occurs but, for a period of time, the
inlet is impassable. This inflow effect occurs year-round.
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Summer Weather
In the summer months (May to September) the frequency and severity of the
storms are much reduced. Low pressure areas usually remain offshore as the Pacific
High strengthens and moves further north. This northward shift causes the main
storm track to shift into the northern Gulf of Alaska and across northern British
Columbia. South of this track, minor frontal systems, upper troughs and thunderstorms produce most of the weather. In August and September, weeks can pass
between weather systems.
Summer fronts
L
Fig. 3-14 - A typical summer pressure pattern shows a front crossing the
coast with an indication of the winds near the front
During the summer months, fronts tend to approach the coast from the northwest
across the Gulf of Alaska. Over the northern coastal areas, the front is usually accompanied by a narrow band of cloud and light rainfall. Southeast winds will tend to
increase along the coast just ahead of the front, then shift into the northwest with its
passage. As the front continues southward and presses further into the Pacific High,
the rain area often disappears and the clouds begin to dissipate.
As the front moves inland, it is weakened by subsidence to the lee of the Coast
Mountains.
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CHAPTER THREE
Thermal Troughs
THERMAL TROUGH
H
L
Fig. 3-15 - Thermal trough
The usual summer pressure pattern has a high pressure area over the eastern Pacific
and troughs of low pressure in the southern interior of BC This trough forms due to
prolonged heating and is referred to as a thermal trough. The effect of this trough is
to create a light, disorganized wind pattern as air flows toward the hottest locations.
Along the coast, light winds in the morning are replaced by strengthening inflow
winds during the afternoon and evening in most inlets and valleys, as the cool coastal
air is drawn towards the interior. Locations in BC with marked inflow winds are the
Strait of Juan de Fuca, Portland Inlet, Howe Sound, through the Hope area and up
the Fraser Canyon.
On occasion, during the summer the thermal trough will move out from the interior
and onto the coast. When it does, it normally lies over Georgia Strait causing subsident outflow winds from the interior which gives clear skies and light winds along the
coast.
After a few days, the thermal trough will shift back into the interior causing
westerly winds of 20 to 30 knots through Juan de Fuca Strait and into southern
Georgia Strait. Sea fog and stratus will accompany these winds and may extend as far
as Vancouver Airport and Boundary Bay. In the strongest cases, the fog will lift into
stratus and spread up the Fraser Valley to Hope.
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Cold Lows
A cold low is a large, nearly circular area of the atmosphere in which temperatures
get colder toward the centre of the low, both at the surface and aloft. While a surface
low pressure centre is usually present beneath the cold low, its true character is most
evident on upper charts. The significance of cold lows is that they produce large areas
of cloud and precipitation, tend to persist in one location for prolonged periods of
time and are difficult to predict.
Cold lows can occur at any time of the year, but the most frequent occurrence,
know as “cold low season,” is from the end of May to mid-July. At this time, pools of
cold air break away from the Aleutian Low and move southeastwards to take up a
nearly stationary position off the British Columbia or Washington coast. Once established, the cold low will generate a series of upper cold fronts which will rotate across
southern British Columbia. The overall effect is to produce a widespread area of cool,
unstable air in which bands of cloud, showers and thundershowers occur. Along the
deformation zone to the northeast of the cold low, the enhanced vertical lift will
thicken the cloud cover and can produce widespread precipitation. In many cases, the
deformation zone is where widespread and prolonged thunderstorm activity occurs.
Eventually, a strong system will approach from the west and will have sufficient
strength to force the cold low inland, usually in the form of a strong upper trough. A
favourite track is across the northern United States, but alternate tracks are across
southern British Columbia or even northeastward along a line from Seattle to Fort
St. John. As it crosses the area, widespread cloud, showers, thundershowers, or even
steady rain can occur for a period of 24 to 48 hours. Normally, while the original cold
low is moving off, the next one is already moving across the Gulf of Alaska on its way
to take up residence off the coast.
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CHAPTER THREE
H
L
Surface Analysis
H
H
L
850 hPa Analysis (about 5,000 feet)
H
L
L
H
L
Fig. 3-16 - Typical surface and upper level pattern for a cold low event
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Notes:
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CHAPTER FOUR
Table 3: Symbols Used in this Manual
Fog Symbol (3 horizontal lines)
This standard symbol for fog indicates areas where fog is frequently observed.
Cloud areas and cloud edges
Scalloped lines show areas where low cloud (preventing VFR flying) is known to
occur frequently. In many cases, this hazard may not be detected at any
nearby airports.
Icing symbol (2 vertical lines through a half circle)
This standard symbol for icing indicate areas where significant icing is relatively
common.
Choppy water symbol (symbol with two wavelike points)
For float plane operation, this symbol is used to denote areas where winds and
significant waves can make landings and takeoffs dangerous or impossible.
Turbulence symbol
This standard symbol for turbulence is also used to indicate areas known for
significant windshear, as well as potentially hazardous downdrafts.
Strong wind symbol (straight arrow)
This arrow is used to show areas prone to very strong winds and also indicates the
typical direction of these winds. Where these winds encounter changing topography
(hills, valley bends, coastlines, islands), turbulence, although not always indicated,
can be expected.
Funnelling / Channelling symbol (narrowing arrow)
This symbol is similar to the strong wind symbol except that the winds are constricted
or channeled by topography. In this case, winds in the narrow portion could be very
strong while surrounding locations receive much lighter winds.
Snow symbol (asterisk)
This standard symbol for snow shows areas prone to very heavy snowfall.
Thunderstorm symbol (half circle with anvil top)
This standard symbol for cumulonimbus (CB) cloud is used to denote areas
prone to thunderstorm activity.
Mill symbol (smokestack)
This symbol shows areas where major industrial activity can impact on aviation
weather. The industrial activity usually results in more frequent low cloud and fog.
Mountain pass symbol (side-by-side arcs)
This symbol is used on aviation charts to indicate mountain passes, the highest point
along a route. Although not a weather phenomenon, many passes are shown as they
are often prone to hazardous aviation weather.
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Chapter 4
Seasonal Weather and Local Effects
Introduction
10,000 FT
7000 FT
5000 FT
3000 FT
2000 FT
1500 FT
1000 FT
WATSON LAKE
600 FT
300 FT
DEASE LAKE
0 SEA LEVEL
FORT NELSON
WARE
INGENIKA
MASSET
SANDSPIT
PRINCE RUPERT
TERRACE
SMITHERS
BELLA BELLA
FORT ST JOHN
MACKENZIE
PRINCE GEORGE
PUNTZI MOUNTAIN
PORT HARDY
VALEMOUNT
WILLAMS LAKE
CAMPBELL RIVER
COMOX
KAMLOOPS
TOFINO
GOLDEN
LYTTON
NANAIMO
VICTORIA
VERNON
KELOWNA
FAIRMONT
PENTICTON
CASTLEGAR
CRANBROOK
Map 4-1 - Topography of GFACN31 Domain
This chapter is devoted to local weather hazards and effects observed in the
GFACN31 area of responsibility. After extensive discussions with weather forecasters,
FSS personnel, pilots and dispatchers, the most common and verifiable hazards are
listed.
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CHAPTER FOUR
Most weather hazards are described in symbols on the many maps along with a
brief textual description located beneath it. In other cases, the weather phenomena are
better described in words. Table 3 (page 74 and 207) provides a legend for the
various symbols used throughout the local weather sections.
South Coast
10,000 FT
7000 FT
5000 FT
3000 FT
2000 FT
PORT HARDY
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
CAMPBELL RIVER
COMOX
PEMBERTON
TOFINO
VANCOUVER
NANAIMO
HOPE
ABBOTSFORD
VICTORIA
Map 4-2 - South Coast
For most of the year, the winds over the South Coast of BC are predominately from
the southwest to west. During the summer, however, the Pacific High builds northward over the offshore waters altering the winds to more of a north to northwest flow.
Regardless of the direction, the coastal area is exposed to every weather system
approaching off the Pacific Ocean. On the plus side, it is the bulk of Vancouver Island
that takes the brunt of these storm, ameliorating their effect on the inner waters and
mainland areas. Compounding, the problem is the mountainous terrain, that rises
abruptly almost from the waters edge, ensuring each system undergoes immediate
upslope lift. The question for meteorologists and pilots then becomes not “Will we
get precipitation?” but “How much precipitation will we get and what type?”
The answer to these questions lies in the seasons of the year.
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(a) Summer
Summer over the South Coast tends to be fairly benign, especially when compared
to a typical winter. Frontal systems do make an appearance from time to time, but for
the most part they make little impression. Approaching from the Gulf of Alaska, but
with little in the way of cold air to feed their development, these systems develop
slowly and are relatively weak. Typically, a band of cloud and light precipitation over
the northern end of Vancouver Island may dissipate to just broken cloud and showers in the south.
Behind these fronts, a ridge of high pressure will build towards the coast. Rising
pressures ahead of this ridge will then give a period of brisk northwest winds to the
coast. The strongest northwest winds are often reported where the air stream is funnelled between the mountains of the mainland and Vancouver Island. This effect is
particularly noticeable in the spring months, as the fronts still retain some of the
strength of winter storms.
Traditionally, the latter parts of May and June tend to be a cloudy and wet. During
this time, a series of cold lows spawn over the northern Gulf of Alaska and move
southward along the coast. Their tracks are difficult to predict but a favourite path is
along the west coast of Vancouver Island and then inland, either through the Juan de
Fuca Strait or northern Washington State. Either track produces widespread cloud,
showers and cool temperatures. Only when the Pacific High builds far enough
northward is this pattern cut off.
Although not frequent, thunderstorms do occur along the coast of British
Columbia in the summer. Air mass thunderstorms are the most common, tending to
develop during the late afternoon or evening and drifting eastward along the sides of
inlets or valleys. Although short-lived, they can produce intense hail and lightning.
On occasion, frontal thunderstorms will move into the coastal areas. One favourite
track is eastward through Juan de Fuca Strait, or northward through Washington,
ahead of an approaching upper trough of low pressure.
The scourge of summer over the South Coast is the sea fog and marine stratus that
often lies over the cold offshore waters, near the western entrance of Juan de Fuca
Strait. When inflow conditions occur, the fog and stratus are frequently drawn into
the strait, penetrating as far as the Victoria area. Since inflow is common, fog is
frequently found in Juan de Fuca Strait for much of the year.
Many of the mainland valleys experience inflow winds in the summer due to
the intense heating of the air over the Southern Interior. As pressures fall over the
interior, a flow of cool, moist air begins to flow inland. However, as the thermal
trough is forced inland, these inflow winds will occur on a daily basis, drawing low
marine cloud into the inner coastal areas and even the mainland valleys.
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CHAPTER FOUR
(b) Winter
Winter is a much more dramatic season. The Pacific High retreats with the sun
allowing the Aleutian Low to move back into the Gulf of Alaska. As it does, a strong
north to south temperature gradient is established that will provide the energy for
significant development of approaching weather systems.
The usual pattern is for a deep low-pressure system to approach from the west to
southwest, accompanied by an occluding frontal system. Ahead of the approaching
low, a warm front will spread extensive, deep-layered cloud, steady precipitation and
strong southeast winds across the area. With the passage of the warm front, the
precipitation becomes intermittent or ends completely, the lowest cloud layers break
up and the winds shift to a moderate to strong south to southwest direction. The
arrival of the trailing cold front is marked by the development of showers, which can
be both widespread and heavy in intensity. Finally, behind the cold front, strong gusty
northwest winds often prevail. With the most active fronts, warm or cold, the winds
can briefly rise to 60 knots with gusts above 80 knots at locations over the northern
end of Vancouver Island. Of the two, the southeast winds tend to be the strongest,
but they both can be very strong where channelling and coastal convergence occurs.
One particularly wet pattern, known as the “Pineapple Express,” occurs when a
deepening trough of low pressure offshore causes the frontal system to stall over the
South Coast. Eventually, moist, tropical air is picked up by the upper winds from near
the tropics and carried northeastward onto the BC coast. The result is extremely
heavy precipitation events where local rainfall amounts of 100 millimetres or more
can occur..
Photo 4-1 - Pineapple Express
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Air mass thunderstorms occur most often during the winter, especially in the very
cold air that follows the passage of a cold front. This cold air is heated by the ocean
and becomes very unstable. As it does, lines of towering cumulus and cumulonimbus
cloud develop which then move toward the coast to arrive about 12 to 24 hours after
the frontal passage.
Surface high pressure systems, long valued for their generally good weather, are
stronger in the winter than in the summer. Along the coast, it is these ridges of high
pressure that provide the only break between active weather systems. As the ridge of
high-pressure approaches the coast, the higher level clouds dissipate and the lower
cloud layer break up leaving scattered to broken convective clouds.
Winter is also the time of “outflow winds”. Although infrequent, if the cold air
deepens sufficiently over the interior of British Columbia, it can flow through the
coastal passes and along the Fraser Canyon to spill out onto the South Coast through
the inlets and valleys. Depending on the weather pattern, this outflow condition can
persist for days without respite. The most common case will see the cold air cover the
northern half of Vancouver Island but stall over the warmer waters of the Strait of
Georgia. Only in the strongest outflow situation will the cold air completely cover
Vancouver Island.
Along the mainland coast, a band of cloud and showers or flurries will accompany
the leading edge of the outflow, to be followed by clearing skies. These clear skies
will then persist throughout the period of outflow winds. Offshore, the cold dry air
flowing over bodies of water, such as the Strait of Georgia or Johnstone Strait, will
usually become unstable and will pick up enough moisture to give heavy snow showers
along the east side of Vancouver Island.
The strong winds funnel down the mainland inlets often reaching speeds of 25 to
35 knots and occasionally rising as high as 50 knots. When the strong winds flow out
from the mouths of the inlets they continue for some distance but gradually fan out
and weaken. Extreme caution is advised when crossing coastal inlets during an outflow situation as the winds could increase very abruptly in a narrow band near the
mouth of the inlet. One hint is to watch for the telltale ripple or wave pattern produced by these winds.
arrival of warmer air being driven ahead of an approaching storm. With cold air along
the coast, the precipitation may start as snow but changes to either rain, or rain and
snow mixed, as temperatures moderate. Freezing rain can also occur in the deeper
mainland valleys and inlets (including the eastern end of the valley) until the cold air
is fully scoured out by the approaching warmer air.
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CHAPTER FOUR
(c) Local Effects
PORT
East Coast of Vancouver Island – Victoria to Nanaimo
ALBERNI
SW
BAMFIELD
NANAIMO
PACHENA
STRAIT OF
GEORGIA
COWICHAN LAKE
VANCOUVER
NITINAT LAKE
LADYSMITH
SE
CARMANAH
SW
BOUNDARY
BAY
DOWNDRAFTS SALTSPRING
ISLAND
SE
10,000 FT
7000 FT
5000 FT
3000 FT
2000 FT
1500 FT
SHERINGHAM
POINT
DISCOVERY ISLAND
VICTORIA
1000 FT
600 FT
300 FT
0 SEA LEVEL
JUAN DE FUCA TRIAL ISLAND
STRAIT
Map 4-3 - East Coast of Vancouver Island – Victoria to Nanaimo
Summer and autumn weather is more conducive to flying; the main problems this
time of year are sea stratus through Juan de Fuca Strait in the summer and sea fog in
the fall. The San Juan Islands are more prone to morning fog than the Gulf Islands.
The top of the fog usually lies between 1,000 and 2,000 feet AGL while the sea stratus tends to produce ceilings between 1,500 and 2,500 feet AGL. A typical pattern is
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for the stratus to move inland over succeeding days and become more and more reluctant to dissipate. Stratus typically lies just off Victoria Airport but covers the Gulf
Islands. Fog banks off the east end of the runway at Victoria can be blown in as pulses by the sea breeze.
At the Duncan Airport, the runway is oriented 12-30 and lies uphill to the northwest. Westerly winds through Cowichan Valley often turn southward along the runway toward the valley and accelerate (15 knot wind becomes 25 knots). This downflow can be up to 500 ft/min resulting in aircraft landing short.
There are large hills on Pender Island where all winds, except southerlies, flow over
hills resulting in downflow winds over the runway.
Nanaimo Airport is situated in a valley with local mountaintops ranging from 700
feet to 4,800 feet. While protected from the stronger surface winds found over the
open water, it is susceptible to low-level mechanical turbulence and wind shear when
winds aloft are strong. Most common during winter storm conditions, these hazards
can be severe. Victoria airport, while somewhat protected from the common southeasterly winds, is more exposed to southerly and southwesterly winds and can experience hazardous wind shear when significant winds aloft are from a conflicting direction.
In addition to the regular METARS, it is worthwhile for low flying traffic over the
water to check the marine wind reports from Trial Island just the south of Victoria,
Victoria Harbour, Discovery Island east of Victoria Harbour at the south end of Haro
Strait, Kelp Reef in Haro Strait east of Victoria Airport, Saturna Island near Active
Pass in the Gulf Islands, and Entrance Island north of Nanaimo. Though not certified by Transport Canada for aviation weather, these sites, taken as a group, give a very
good indication of the surface winds over the water.
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CHAPTER FOUR
Vancouver Area Including Pitt Meadows, Langley and Boundary Bay
10,000 FT
7000 FT
SQUAMISH
5000 FT
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
HOWE
SOUND
300 FT
0 SEA LEVEL
STRAIT OF
GEORGIA
PITT LAKE
INDIAN ARM
ALOUETTE
LAKE
VANCOUVER
SE
PITT MEADOWS
LANGLEY
POINT ROBERTS
SE
STAVE
LAKE
ABBOTSFORD
BOUNDARY BAY
Map 4-4 - Vancouver Area Including Pitt Meadows, Langley and Boundary Bay
As most systems move over the south coast from the west or southwest (upper flow,
not the surface winds), the north side and east end of the Fraser Valley tend, due to
upslope, to get greater amounts of cloud and precipitation producing widespread areas
of low flying conditions. When icing is indicated, you can expect it to be significantly heavier over the north shore mountains from Powell River to Hope. This includes
not only the usual mixed icing in the convective build-ups associated with cold fronts,
but also moderate and heavy rime icing with the onset of warm fronts. Pilots report
some of the worst icing conditions they’ve ever experienced occur in this area.
Because of this, most choose to ascend to the west over the Strait.
A general sense of the local weather can usually be obtained by making reference
to the local METARS as well as the one from Bellingham (KBLI). Radiation fog is
common around Langley and Pitt Meadows in the fall due to the low, flat bog land.
Abbotsford Airport, however, is seldom a problem. Vancouver and Boundary Bay, on
the other hand, are very susceptible to marine stratus/fog advection coming off the
straits. This can cause conditions to deteriorate very rapidly. It most commonly occurs
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in the early morning with inflow conditions [usually with a ridge along the coast], and
can last from an hour or so to several hours, sometimes continuing into mid afternoon. When conditions persist over several days, the low cloud/fog tends to persist
longer each day.
Strong winds at Vancouver are usually from either the west or southeast with the
southeasterlies tending to be gusty and turbulent. West winds can reach speeds
exceeding 40 knots on occasion but tend to be stable and relatively smooth.
Tidal currents in the Fraser River can be hazardous for landing seaplanes, especially with northwest winds that oppose the current, producing steep, choppy waves.
Similar or worse conditions are observed in the Pitt River, all the way to Pitt Lake.
Abbotsford to Hope
LYTTON
MERRITT
PITT LAKE
TOLL
BOOTH
ALOUETTE
LAKE
HARRISON
LAKE
LANGLEY
SW
STAVE
LAKE
ABBOTSFORD
CHILLIWACK
10,000 FT
HOPE
COQUIHALLA
HIGHWAY
7000 FT
PRINCETON
5000 FT
3000 FT
ALLISON
PASS
2000 FT
1500 FT
1000 FT
600 FT
300 FT
COMMON FLIGHT
ROUTES
0 SEA LEVEL
Map 4-5 - Abbotsford to Hope
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CHAPTER FOUR
The worst weather in the Fraser Valley occurs from September to April and is usually associated with precipitation that brings on rapidly lowering ceilings and visibility. The low cloud tends to thicken noticeably near Agassiz and worsens as you go
eastward into the mountains. In a southwesterly flow, poor weather piles up along the
mountains, with higher ceilings on the south side of the valley.
During times of strong northwest winds over the Strait of Georgia, the winds will
curl into a strong westerly along the valley. Westerlies of 25 gusting to 45 knots with
stronger gusts are common behind cold fronts. If the air mass is cold, snowshowers
and very poor visibility will be found along the high ground that rims the eastern end
of the valley. During times of strong inflow, anticipate significant mechanical turbulence on the south side of the Fraser River near Hunter Creek, located west of Hope
between Laidlaw and Flood. During times of strong outflow the weather will generally be clear, but significant turbulence should be anticipated around Chilliwack and
Sumas Mountains below 5,000 feet ASL.
The summer sea breeze channels between Chilliwack and Hope and can be quite
strong (20 to 35 knots). Smog (top of layer 1,500 to 2,000 feet) can reduce visibility
to 2 to 3 miles at the east end of the valley during warm, summer afternoons. When
approaching from higher levels, reflection of sunlight off the smog can make it difficult to view the ground.
Very strong winds are common over Harrison Lake and Harrison River in summer.
Over the lake, the worst winds are northerly sea breezes during summer afternoons.
These can produce significant turbulence and waves as high as 4 feet.
Low cloud and poor visibility are facts of life in Hope from late fall to late spring.
Only during times of dry, cold outflow will the cloud clear from the area. Turbulence
is common in the Hope area when strong inflow or outflow winds are occurring.
Inflow is common year round while outflow tends be a winter phenomenon. Local
pilots report that this turbulence is usually restricted below 3,000 feet AGL. During
the winter months, icing in cloud is often significant to the east and northeast of
Hope, along the Coquihalla to Merritt route. When checking the local METARS
note that Hope Slide is about 10 to 15 miles east of Hope and about one thousand
feet higher up.
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Vancouver to Pemberton along Howe Sound
JERVIS INLET
PEMBERTON
WHISTLER
LILLOOET
LAKE
DAISY LAKE
SECHELT INLET
10,000 FT
7000 FT
5000 FT
3000 FT
2000 FT
GAMBIER
ISLAND
SQUAMISH
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
COMMON FLIGHT
ROUTES
POINT
ATKINSON
VANCOUVER
SE
PITT MEADOWS
HARRISON
LAKE
Map 4-6 - Vancouver to Pemberton along Howe Sound
There is a high volume of traffic along this route throughout the year. It is possible
ABBOTSFORD
for pilots to contact others on VHF to determine the weather ahead. However, when
the conditions are marginal and pilots fly at low altitudes, radio contact is limited by
CHILLIWACK
the terrain to a very small area ([usually only a dozen miles ahead, often
much less).
Often used as an alternate route to the interior, this route follows Howe Sound
north from Vancouver to Squamish and then follows the Cheakamus River valley to
Whistler and Pemberton. Just north of Squamish the valley splits into three valleys,
with the largest, the Squamish River valley, opening to the northwest. Experienced
pilots tell of frequent navigation errors near Squamish, especially in bad weather.
At the mouth of Howe Sound, near Vancouver, channelling causes outflow winds
to increase dramatically. Burrard Inlet runs east-west and meets Howe Sound at right
angles just northwest of Vancouver in the Strait of Georgia. When the outflow are
strong through Howe Sound and Burrard Inlet, expect at least moderate mechanical
turbulence in the immediate vicinity. The winds over the open waters of the southern
Strait of Georgia are also often in conflict. When the outflow from Howe Sound is
strong and there is a strong southeasterly wind blowing along the Strait, these winds
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are meeting each other almost head on. Add to this the fact that the upper winds just
off the surface are commonly south or southwesterly winds aloft producing a hazardous condition with mechanical turbulence at lower levels and shear turbulence just
above the turbulent boundary level winds, starting anywhere from 800 to 2,000 ft
MSL. This area lies on busy float plane routes in and out of Vancouver Harbour
enroute the Sunshine Coast and Howe Sound.
In the summer, thunderstorms are not infrequent in Howe Sound and tend to move
up the sound. Strong inflow winds are common into Howe Sound to Squamish, often
exceeding 30 knots. In a strong southeast flow up Howe Sound, there is strong turbulence along the highway where it is cut into the cliffs.
In the winter, during outflow conditions, turbulence to 5,000 feet AGL is common
on the route north of Squamish, especially where the valleys narrow. Local pilots
report that the bad weather tends to pile up on the east side of the river and recommend staying on the left side with the river to your right. The worst location for low
cloud is one to two miles north of Squamish. The low cloud usually ends near Daisy
Lake. A second choke point exists near the bend in the road, to the south of Whistler.
The valley from Whistler to Pemberton is notorious as one of the last to dry out and
clear after frontal precipitation. Also note, there are some very high wires along
Highway 99, which are dangerous for fixed wing aircraft following the river.
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CAMPBELL RIVER
Strait of Georgia – Vancouver - Nanaimo - Powell River - Comox
POWELL
RIVER
COMOX
JERVIS INLET
TEXADA
ISLAND
HORNBY ISLAND
DENMAN ISLAND
SISTERS ISLAND
SECHELT INLET
PORT
ALBERNI SW
LASQUETI ISLAND
QUALICUM BEACH
SW
BALLENAS ISLAND
GIBSONS
STRAIT
OF GEORGA
NANAIMO
GABRIOLA ISLAND
10,000 FT
7000 FT
5000 FT
VANCOUVER
VALDES ISLAND
LADYSMITH
SE
3000 FT
2000 FT
1500 FT
1000 FT
SW
600 FT
300 FT
DOWNDRAFTS
SE
BOUNDARY
BAY
0 SEA LEVEL
Map 4-7 - Strait of Georgia – Vancouver - Nanaimo -Powell River - Comox
Southeasterly winds are the most significant along the strait and they tend to be
gusty. Turbulence along this portion of the strait is generally not significant except for
mechanical turbulence in the vicinity of terrain along the coast and the northern Gulf
Islands. Channelling and coastal convergence of the surface wind can be severe along
either side of Texada Island due to the height of the ridges. Coastal convergence can
produce winds of up to 35 knots in the summer near Qualicum Beach. Nanaimo
Harbour is almost always calm.
Low cloud and fog tend to persist wherever there are hooks of land to hold it, such
as around Nanaimo and between Texada and Lasqueti Islands. In general, fog and low
cloud are more persistent over the northern half of the strait. At Nanaimo, fog is
much more common over the airport than at the floatplane base.
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Powell River can be hazardous in southeast winds with lots of subsiding air near the
runway. Hills in the vicinity rise to approximately 3,500 ft. The lake can be especially bad in southeast winds, which swirl around a point of land on the southern end of
the lake. Although still dangerous at times, it is often calmer on the north end of the
lake, and along the ridge on the west side of the lake.
Once again the unofficial wind reports from the marine bulletins are very useful
indicators of surface wind conditions. See especially Grief Point near Powell River,
Sisters Island off the southwest tip of Texada Island, Ballenas Island near Qualicum
Beach, and Merry Island just offshore from Sechelt.
Inner Straits from Powell River/Comox – Queen Charlotte Sound
QUEEN
CHARLOTTE
SOUND
RIVERS INLET
SE
SE
PORT HARDY
QUEEN
CHARLOTTE
STRAIT
SE
QUATSINO SOUND
MALCOLM ISLAND
ALERT BAY
KNIGHT INLET
NIMPKISH
LAKE
MINSTREL ISLAND
SE
SE
JOHNSON STRAIT
BUTE INLET
SE
KELSEY BAY
TOBA INLET
CHATHAM POINT
SW
CAMPBELL RIVER
10,000 FT
7000 FT
5000 FT
3000 FT
2000 FT
1500 FT
COMOX
1000 FT
POWELL
RIVER
600 FT
300 FT
TOFINO
SUTTON PASS
0 SEA LEVEL
Map 4-8 - Inner Straits from Powell River/Comox – Queen Charlotte Sound
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As with areas further south, weather in this region tends to be relatively dry as
Pacific weather systems are dried as they pass over Vancouver Island. The airport at
Campbell River reports considerably more fog than occurs over the Strait, especially
during autumn months. The Comox Airport, right along the water, is not affected as
much by this fog. Note, however, that if the conditions in the Strait near both Comox
and Campbell River involve ceilings or even significant quantities of scattered cloud
at 400 feet or below, Campbell River, because of its elevation will usually be in fog.
The prevailing southeasterly winds in the winter tend to force low cloud onto the east
coast of Vancouver Island, often leaving the mainland half of the Strait less congested.
Bute Inlet is very susceptible to strong outflow with strong vertical eddies over the
water. Evidence can sometimes be seen in a “catspaw” area with whitecaps. A very turbulent spot, both on the water and in the air, is Seymour Narrows in Discovery
Passage.
In summer, other than the occasional weak frontal system, the main problem is fog,
which tends to sit along the coastal areas and over the straits. This situation is very
common when there is an area of high-pressure building to the west of the island. The
fog and stratus become widespread overnight and generally wrap around the north
end of the island, and extend both northwards along the Central Coast and southwards to Malcolm Island, or even as far as Chatham Point. Heating from the sun will
dissipate the fog and stratus off the land by late morning; however, fog banks will persist over Queen Charlotte Strait throughout the day and begin to expand once the sun
sets. In general, during July and August, conditions tend to be poor in the morning,
good in the afternoon and often decrease rapidly after sunset.
The sea breeze activity is pronounced in this area during the summer. Winds over
the straits and at Port Hardy will be light in the morning and strengthen to northwesterly 15 to 25 knots in the afternoon. Alert Bay airport has an abrupt drop-off
near the northwest end of the runway that induces a downflow in strong southeasterly winds.
Winter storms tend to induce strong southeasterly winds in the straits reaching
typical values of 25 to 45 knots in Johnstone Strait and 35 to 50 knots in Queen
Charlotte Strait. In such cases, moderate to severe mechanical turbulence is common.
Local pilots report that, in a strong east-southeast flow, severe turbulence is common
near Actaeon Sound (about 20 miles northeast of Port Hardy), in Sargeant’s Pass near
Minstrel Island and in Surge Narrows (near Campbell River).
When the upper flow is strong southwesterly (around 50 knots or greater) there is
a tendency for strong channelled winds to come out of Telegraph Bay (near Malcolm
Island). Also in such conditions strong rotor activity develops in Blind Channel
(between Kelsey Bay and Chatham Point) resulting in swirling winds, sheets of water
lifting from the surface and even waterspouts.
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There is a typical weather regime change at Kelsey Bay with cloud and rain becoming more common further north as wind patterns change. One area with consistently poor weather is along Johnstone and Queen Charlotte Strait, with the area between
Chatham and Alert Bay receiving special mention for low hanging cloud.
The ceiling report at Port Hardy is often representative of surrounding land areas,
as long as the differing terrain height is considered. If the ceiling is 500 feet at Port
Hardy, terrain to the west is most likely obscured. In autumn and winter months, the
northern end of the island is often battered by extreme winds reaching 80 knots or
more. In such a case, turbulence is often extremely violent in the vicinity of the
Brooks Peninsula and near Sartine Island in the Scott Islands Group.
Wind speeds over the water are available from the Marine Bulletins. Near
Campbell River see Mudge Island (more indicative of winds at the water aerodrome
at Campbell Spit). At the south end of Johnstone Straits, see Chatham Point, at the
north end, Helmcken Island. From south to north the winds in Queen Charlotte
Sound are found at Pulteney Point, Scarlett Point and Herbert Island. North of Port
Hardy are Pine Island and Egg Island.
RIVERS INLET
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PORT HARDY
West Vancouver Island Including Routes to Port Alberni and Tofino
BROOKS
PENINSULA
NIMPKISH LAKE
SE
JOHNSON STRAIT
KYUQUOT SOUND
SE
SE
KELSEY BAY
S
CHATHAM POINT
NOOTKA
CAMPBELL RIVER
ESTEVAN POINT
COMOX
10,000 FT
SUTTON PASS
TOFINO
LENNARD
ISLAND
PORT
ALBERNI
7000 FT
5000 FT
SW
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
300 FT
LOW
MARINE
CLOUD
BARKLEY
SOUND
CAPE BEALE
BAMFIELD
ALBERNI
INLET
0 SEA LEVEL
Map 4-9 - West Vancouver Island including routes to Port Alberni and Tofino
There are a few common routes across southern Vancouver Island. One follows the
Cowichan Valley from Duncan west past Cowichan Lake to a pass at Nitinat Lake
and thence to the coast. If the ceilings permit it is possible to go north over the mountains from Cowichan Lake directly to Port Alberni, but this route is more commonly used to reach the west coast and points north. Most traffic to Port Alberni takes
the route west from the Parksville/Qualicum area. This climbs over a pass to Port
Alberni
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The route from Parksville to Port Alberni may appear clear until just prior to reaching Port Alberni. Mechanical turbulence is common at the junction of Cowichan
Lake Valley and Nitinat Lake Valley, as well as shears occurring at the east end of
Cowichan Lake during afternoon, when westerly flow along the valley meets onshore
sea breezes. The valley is tight from the southwest end of Cowichan Lake to Port
Alberni with low cloud persisting sometimes at the bend in the road, on the west side
of the knoll. If flying from Alberni down Sprout Lake, light to moderate turbulence
is common near Kennedy River. In a westerly flow, turbulence is frequent over the
ridge near Cameron Lake, as well as the area between Horne and Cameron Lakes. In
the winter, conditions in Sutton Pass can get very low, especially where the road sits
up on the side of the pass. During the winter, snow along the road at the top of the
pass is not uncommon.
Some of the worst weather in British Columbia occurs along the west coast and
western slopes of Vancouver Island. Strong winds, low cloud, heavy precipitation and
fog banks combine to make this a treacherous route year round. Tofino is a favourite
place to visit but low cloud and fog frequently sit off the coast and are quick to move
over the airport. Local pilots say it is essential to have sufficient fuel to return to an
alternate when trying to fly into the area. The observing site at Tofino Airport does
not provide an unobstructed view to the sea and, thus, sea fog may not be seen.
Lighthouse reports at Amphitrite Point, Estevan Point and Lennard Island are
extremely important, especially for float plane activity. If any low cloud is reported at
coastal airports, the west coast of the island is likely closed to visual traffic. From the
south to north, marine reports on the west coast include Sheringham Point,
Carmanah, Pachena Point, Cape Beale, Amphitrite Point, Lennard Island and
Estevan Point.
Near Gold River and Tahsis, inflow winds can make water very rough at the seaplane dock. There are also strong river currents near the dock at Gold River. See the
marine report at Nootka for an indication of winds in Nootka Sound west of Gold
River.
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North Coast
MASSET
PRINCE RUPERT
TERRACE
SANDSPIT
10,000 FT
7000 FT
5000 FT
3000 FT
2000 FT
BELLA BELLA
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
Map 4-10 - North Coast
The predominate circulation over the North Coast is from the west. As in the
south, the coastal area is fully exposed to every weather system approaching off
the Pacific Ocean, and once again the terrain rises abruptly from the water’s edge
ensuring each system immediately undergoes strong upslope lift. If one observation
could be made, it would be that seasonally the North Coast weather tends to be more
prolonged and nastier than the southern section. Most of this can be attributed to the
fact that the northern storm track tends to lie over this area for much of the year and
the fact that there is no sheltering bulk of Vancouver Island to weaken the incoming
storms. VFR flight is possible in this area any time of the year, but the combination
of rapidly changing weather and a lack of alternate airports can ensnare the unwary.
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(a) Summer
During the summer months, fronts tend to approach the coast from the northwest
across the Gulf of Alaska. Over the coastal areas, a band of cloud and light rain usually accompanies the front. Winds will tend to increase along the coast just ahead of
the front and can be expected to reach values of 30 to 40 knots over some exposed
locations around the Queen Charlotte Islands.
Behind the front, pressure rises are fairly strong as the following ridge of high pressure builds towards the coast. This produces strong northwest winds following the
frontal passage. The strongest northwesterly winds are often reported along the west
coast of the Queen Charlottes.
Although not frequent, thunderstorms do occur over the inland sections of the
North Coast in the summer. Air mass thunderstorms are the most common, tend to
develop during the late afternoon or evening, and drift eastward along the sides of
inlets or valleys. On rarer occasions, frontal thunderstorms will move into the coastal
areas.
Sea fog and marine stratus are prevalent over the offshore waters just to the west of
the Queen Charlottes and is easily advected into the coastal areas. Once there it will
tend to dissipate or thin during the day but then be quick to reform again once darkness falls. This pattern will continue until some kind of pressure system moves drier
air into the region.
(b) Winter
Along the coast, approaching frontal systems provide for dramatic weather. Ahead
of the warm front, extensive, deep-layered cloud will give steady heavy precipitation
and strong southeast winds across the area. With the passage of the warm front, the
precipitation becomes intermittent, the lowest cloud layers break up somewhat but
the higher cloud layers will persist. At the same time, strong southerly winds usually
persist. Frequent showers of rain and ice pellets mark the arrival of the trailing cold
front, which can be both widespread and heavy in intensity. Strong west to northwest
winds usually prevail, especially around the Queen Charlottes and Dixon Entrance.
With the most active fronts, warm or cold, the winds can briefly rise to 60 knots with
gusts above 80 knots at locations around the Queen Charlotte Islands.
Air mass thunderstorms occur most often during the winter in the very cold air that
follows the passage of a cold front. They often move across the area as squall lines
about 12 to 24 hours after the frontal passage.
The only breaks in winter are when ridges of high pressure move across the area,
and these tend to be of short duration. As the ridge approaches, the skies tend to clear
leaving areas of scattered to broken convective clouds. Usually there has to be a trough
to get thunderstorms.
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If the cold air deepens sufficiently over the interior of British Columbia, it can flow
through the coastal mountains, down the coastal inlets, and cascade out over the
coastal waters far enough to cover the Queen Charlotte Islands. This outflow condition can persist for days without respite. The strong winds funnel down the mainland
inlets often reaching speeds up to 60 knots and occasionally rising as high as 100
knots. Side tributaries from the main inlets also have strong winds and, where a major
side valley joins the main inlet, chaotic conditions are found. When the strong winds
flow out from the mouths of the inlets, they continue for some distance but gradually
weaken as the flow is no longer confined. Extreme caution is advised when crossing
coastal inlets during an outflow situation as the winds can increase very abruptly in a
narrow band near the mouth of the inlet.
Outflow conditions bring some of the best flying conditions along the North Coast
during the winter. After the initial surge of cold air, accompanied by a band of cloud
and flurries, the skies will clear and remain so throughout the period of outflow
winds. Offshore, the cold dry air flowing over bodies of water, such as Hecate Strait,
will become unstable and pick up enough moisture to give the potential of heavy
snowshowers along the east side of the Queen Charlotte Islands.
arrival of warmer air being driven ahead of a storm approaching from the Pacific
Ocean. The precipitation with the system frequently starts as snow but changes to
either rain, or rain and snow mixed, over the Queen Charlottes and along the coast
as temperatures moderate. In the inlets; however, the cold outflow winds will
gradually ease as the southeast winds strengthen along the coast. Often rain will be
reported at the mouths of the inlets while snow continues further up in the inlet.
Freezing rain can also occur in the inlets until the cold air is fully scoured out by the
approaching warmer air.
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(c) Local Effects
Northern Vancouver Island to McInnes Island
SE
PRICE ISLAND
MCINNES ISLAND
QUEEN CHARLOTTE
SOUND
IVORY
BELLA BELLA
BURKE CHANNEL
FITZ HUGH SOUND
SE
ADDENBROKE
NW
RIVERS INLET
10,000 FT
GOOSE BAY
(NAMU)
EGG ISLAND
7000 FT
SE
5000 FT
3000 FT
2000 FT
SE
1500 FT
1000 FT
600 FT
300 FT
PORT HARDY
SE
0 SEA LEVEL
Map 4-11 - Northern Vancouver Island to McInnes Island
The scenery along the Central Coast is considered to be some of the most beautiful in British Columbia. Like the Port Hardy area, the weather in this region varies
strongly between summer and winter, and the combination of widely scattered airports, lack of information and the speed at which the weather can change makes this
a treacherous place to fly. All pilots who regularly fly this area make the same suggestions - wait for the breaks in the weather, carry lots of fuel and have a viable plan
on where to go if the weather closes in.
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In summer, the main pattern is the persistence of low cloud and fog banks all along
the coast from Prince Rupert to Port Hardy. These fog banks will move offshore with
daytime heating but are capable of moving back in abruptly and without warning.
Usually the fog will only close the mouths of the inlets, but in the extreme cases the
entire inlet will fill up.
A strengthening afternoon inflow is common in the channels so that by late afternoon the winds are frequently strong and the water too rough to allow floatplanes to
land at the head of inlets.
A couple of places have made a name for themselves. Local pilots suggest you stay
at least 5 miles clear of the Goose Bay area (Namu) and the big hill on the south side
of the mouth to Rivers Inlet when strong southeast winds are blowing (for an indicator of wind strength check the report for Addenbroke). Lee waves are present over
top of Rivers Inlet and flight near 500 feet will be extremely rough in windy conditions.
Very strong outflow conditions are common when pressures build over the interior
and values in excess of 50 knots are not uncommon near Cathedral Point, in the
Burke Channel. These winds are the best indicator of inflow/outflow conditions on
the central coast and are worth monitoring on a regular basis. Outflow of cold air will
also occur ahead of approaching systems, and in the winter this creates the potential
for freezing rain.
The weather report from Bella Coola cannot be used as representing Bella Bella.
Bella Coola lies about 70 miles east at the end of an inlet, which penetrates deep into
the Coastal Range. Bella Bella, the more exposed marine site, reports much more low
cloud and fog. The marine reports from Dryad Point and Ivory Island are more
indicative of conditions near Bella Bella. Westerly winds tend to produce severe turbulence over portions of Rivers Inlet, due to the steep bluffs on the west side of the
inlet. Sea fog is very common over Queen Charlotte Sound. The lighthouse report
from Egg Island is very representative for conditions along the mainland coast. For
Rivers Inlet see the report from Addenbroke at the south end of Fitz Hugh Sound.
There is also a report from McInnes Island further north.
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CHAPTER FOUR
Bella Bella to Prince Rupert
PRINCE RUPERT
LAWYER ISLAND
PORCHER
ISLAND
TERRACE
BONILLA
ISLAND
SE
BANKS
ISLAND
GRENVILLE
KITIMAT
CHANNEL
DOUGLAS CHANNEL
PITT
ISLAND
NANAKWA SHOAL
PRINCESS
ROYAL
ISLAND
SE
10,000 FT
7000 FT
5000 FT
ARISTAZABAL
ISLAND
3000 FT
2000 FT
BOAT
BLUFF
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
PRICE ISLAND
MCINNES
ISLAND
IVORY
BELLA BELLA
Map 4-12 - Bella Bella to Prince Rupert
BURKE CHANNEL
BELLA COOLA
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As with most other regions of the coast, weather hazards differ from summer to
winter. In winter, Pacific low pressure systems can produce very strong winds, often
exceeding 50 knots. When winds exceed 25 knots, moderate or greater turbulence can
be expected around virtually all terrain. Channelling and funnelling can produce
extreme winds in most inlets. The severity of these winds should not be underestimated. Every winter on the north coast there are occasionally winds extreme enough
to induce swirls that can lift water frrom the surface. Note, however, that even when
winds reach 35 knots or more in Chatham Sound (see Lucy Island), Seal Cove can
be relatively calm.
When the strong southeasterly or southerly winds are blowing, experienced pilots
flying between Prince Rupert and the central coast will avoid the inside routes such
as Grenville Channel, and take the longer route outside the islands to avoid the heavy
turbulence induced by the narrows and terrain. The strong northeasterly outflow
winds, common in the winter months, also cause significant turbulence near similar
terrain features. The Portland Inlet, just north of Prince Rupert, the mouth of the
Skeena River and the Douglas Channel further south between Kitimat and the coast,
are particularly affected by outflow winds, often causing turbulence prohibitive of all
small aircraft flight. In the summer strong northwesterly winds can cause moderate
to severe turbulence and rough sea conditions near Port Simpson just south of the
mouth of the Portland Inlet. For indicators of the above conditions see the winds at
Grey Island for the mouth of Portland Inlet and Port Simpson, and at Holland Rock
for the mouth of the Skeena River. The Nanakwa Shoal winds in the buoy report are
indicative of the winds in the Douglas Channel.
Winter systems also bring a lot of precipitation to the north coast. Low ceilings and
poor visibilities are a fact of life for much of the winter. Low cloud tends to bunch up
against Hayes Mountain and lie over the city of Prince Rupert and the airport on
Digby Island. Seal Cove, however, is often less affected and will enjoy somewhat
higher ceilings and visibility. Marine weather reports are available for Green Island
and Triple Island off shore Prince Rupert in Chatham Sound and for Bonilla Island
further south in Hecate Strait.
In summer, two factors dominate local weather: sea fog and afternoon inflow sea
breezes. The inflow can reach 40 knots in some inlets. Fog is most prevalent in August
and early September. Throughout the summer, the fog can sit over the sea, expanding right to the head of the inlets overnight and retreating during the day. Persistent
fog often lies just west of Digby Island and can intermittently roll in and out over the
runway or approaches. Fog is more common over Port Edward and Digby Island than
over Seal Cove.
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CHAPTER FOUR
Central Coast to the Interior Plateau
10,000 FT
7000 FT
5000 FT
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
BURKE CHANNEL
BELLA COOLA
300 FT
0 SEA LEVEL
ANAHIM LAKE
COMMON FLIGHT
ROUTES
CHARLOTTE LAKE
TATLA LAKE
CHILKO
LAKE
KNIGHT INLET
BUTE INLET
Map 4-13 - Central Coast to the Interior Plateau
There are several things that can be said about these routes in general. Firstly, they
go from wet coastal weather near sea level between some of the highest peaks in the
Coastal Range to dry belt weather on the Interior Plateau in a relatively short distance. Secondly, they are all susceptible to inflow and outflow influences. The coastal
inlets and valleys are prone to low cloud during and after precipitation. Inflow winds
tend to push this cloud up the valleys toward the mountains. Outflow winds on the
other hand bring drier air, but at times can be strong enough to cause significant
mechanical turbulence particularly near narrows and rough terrain. Thirdly, these
routes weave through a maze of rivers, valleys and passes. In poor weather conditions
they lend themselves to navigational errors which are all too often fatal. Pilots who
are not experienced and very familiar with their route and its terrain, should not
attempt it in low or even marginal conditions. This hazard is increased for west bound
flights as the weather conditions encountered from the crest of the mountains west
are often considerably poorer than those experienced at the same time on the drier
eastern side. Lastly, there is virtually no weather reporting sites anywhere along these
routes and conditions can, and often do, change rapidly.
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(a) Burke Channel to Anahim Lake
The Bella Coola Valley frequently has fog or low cloud that can persist all day during the fall and winter seasons. When present, this low cloud tends to persist all the
way from Bella Coola to the crest of the Coast Mountains. There is a highway along
this route, but it is not recommended due to its higher elevation. The preferred route
is to follow the Hotnarko and Atnarko River inland due to their lower elevations.
In the winter, high pressure will often dominate over the interior giving outflow
conditions. This can produce significant downslope winds and turbulence over eastern sections of the Bella Coola Valley.
(b) Bute Inlet to Chilko Lake/Tatla Lake
This route follows the Homathko River through the mountains from Bute Inlet. It
is a common route for pilots flying to and from the south coast. Again, low cloud at
the southwest end of the route and mechanical turbulence in strong outflow winds,
are the most common problems.
(c) Knight Inlet To Nimpo Lake/Anahim Lake
This route follows the Klinaklin River from the Knight Inlet to the interior. The
same problems with low cloud and outflow turbulence found on the other routes also
pertain here. However the terrain along this route involves higher passes than the
other routes, and thus this route requires even fairer weather to be flyable.
Coastal lighthouse station reports are available and are especially useful for
conditions near the mouths of the inlets. The automatic weather station at Cathedral
Point in the Burke Channel is the best indicator of inflow/outflow winds along the
central coast.
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TERRACE
PRINCE RUPERT
Bella Bella to Kitimat (Douglas Channel)
SE
KITIMAT
DOUGLAS CHANNEL
NANAKWA SHOAL
SE
10,000 FT
7000 FT
5000 FT
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
BELLA BELLA
BURKE CHANNEL
BELLA COOLA
Map 4-14 - Bella Bella to Kitimat (Douglas Channel)
When active storms are moving northward along the Central Coast, there may be
rotor turbulence on the east side of the valley, due to the higher southeasterly storm
winds interacting with the generally north to south terrain. This turbulence is often
severe if a strong southerly inflow is blowing. (For an indicator of inflow/outflow
winds see the report from the buoy at Nanakwa Shoal. The worst turbulence is
usually found from mid-mountain to peak levels within one or two miles of the
slopes. Flight conditions will generally be smoother and safer along the west side of
the passage.
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This channel is extremely susceptible to freezing rain as the warm air ahead of
approaching frontal systems overruns the cold air in the valley bottoms.
Prince Rupert to Stewart
STEWART
COMMON FLIGHT
ROUTES
PORTLAND
CANAL
10,000 FT
7000 FT
5000 FT
3000 FT
PORTLAND
INLET
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
N
GREEN
ISLAND
GREY
ISLAND
Y
LE
AL
V
S
AS
NW
PRINCE RUPERT
TERRACE
Map 4-15 - Prince Rupert to Stewart
The Portland Canal opens to the northwest just north of Prince Rupert. It runs
northwest and then turns north into the Coastal Range to Stewart. The canal cuts
through a steady upslope regime from low hills and islands at the coast to peaks either
side of the narrow canal in the north ranging from 5 to 7 thousand feet. It is extremely susceptible to heavy snowfall ahead of an approaching frontal system, followed by
freezing rain as the warm air overruns the cold air in the valley bottoms.
Significant turbulence is observed with northwesterly winds to the north of Seal
Cove. The channel at Kincolith experiences severe turbulence in inflow and outflow
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CHAPTER FOUR
patterns. The Portland Canal to Stewart is also often very rough, especially in winter
outflows. Above the highest terrain height, this mechanical turbulence is reduced.
For indicators of these conditions see the winds at Grey Island at the mouth of
Portland Inlet.
Queen Charlotte Islands
LANGARA
DIXON
ENTRANCE
MASSET
KINDAKUN
ROCKS
ROSESPIT
GRAHAM
ISLAND
SKIDGATE
CHANNEL
SANDSPIT
HECATE
STRAIT
MORESBY
ISLAND
SE
10,000 FT
7000 FT
5000 FT
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
300 FT
CAPE ST JAMES
0 SEA LEVEL
Map 4-16 - Queen Charlotte Islands
Though a group of many islands, Graham Island in the north and Moresby Island
in the south make up most of the landmass of the Queen Charlotte Islands. A narrow channel of water, the Skidegate Channel, separates the two main islands. The two
main airports are Sandspit at the northeast corner of Moresby Island and Masset on
the north coast of Graham Island.
The Queen Charlotte Islands exhibit many of the weather phenomena common to
the rest of the North Coast. Winter brings a seemingly endless procession of fronts
off the Pacific accompanied by severe winds, abundant precipitation, low cloud and
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limited visibilities. Hurricane force winds are a frequent feature of winter storms. In
summer the breaks between the systems are longer and sunnier.
Moderate turbulence is often reported to the lee of Tow Hill, east of Masset, during northwesterly or southeasterly winds exceeding 25 knots. Thunderstorms (summer and winter) tend to move through Skidegate Channel on a regular basis, reducing visibility from unlimited to near zero (rain or snow) in a matter of a few minutes.
The only METAR on the islands is at Sandspit. Langara Island and Rose Spit give
winds for the northwest and northeast corners of Graham Island respectively. The
best indicator of conditions in Masset is a limited marine weather report from
Langara in the marine bulletins. Kindakun Rocks gives winds on the exposed west
coast as does Cape St. James on the south tip of Moresby Island.
Prince Rupert to Terrace/Kitimat
10,000 FT
7000 FT
PRINCE RUPERT
5000 FT
3000 FT
2000 FT
TERRACE
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
SE
COMMON FLIGHT
ROUTES
KITIMAT
DOUGLAS CHANNEL
Map 4-17 - Prince Rupert to Terrace/Kitimat
This route follows the Skeena River. With a fall of less than 200 feet, the Skeena
River west of Terrace widens considerably with mountains along either side of the
river rising from 5 to 7 thousand feet. It can be very rough behind the mountain
shoulders along the Skeena River with Telegraph Point renowned for its severe turbulence. Staying over the river can generally reduce the turbulence. When frontal systems ride in from the Pacific, low cloud will persist in this area until well after the
front has passed. As with coastal inlets and valleys in general the inflow conditions
BELLA COOLA
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CHAPTER FOUR
which often arise after frontal passage cause low cloud to build up at narrows or where
there are bends. The region near Salvus can receive extremely heavy precipitation, due
to both convergence at the narrows and lift from the upslope terrain.
With extensive swampy land and other water sources, the valley near Terrace is very
susceptible to fog and low cloud. The airport at Terrace is particularly prone to low
cloud and fog due to its elevation, approximately 500 feet above the town and the valley floor. To the south of Terrace, the mills and smelter at Kitimat can reduce visibility and ceilings, particularly along the west side of the valley.
The Douglas Channel south of Kitimat as well as the valleys south and west of
Terrace are extremely susceptible to freezing rain as the warm air ahead of approaching frontal systems overruns the cold air in the valley bottoms. The strength of the
inflow/outflow winds can be monitored by checking the report from the buoy at
Nanakwa Shoal.
Routes Inland from Terrace
For information on these routes please see the Central and Northern
Interior Section.
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Thompson - Okanagan
GOLDBRIDGE
LILLOOET
CACHE CREEK
KAMLOOPS
LYTTON
SALMON ARM
MERRITT
FALKLAND
VERNON
KELOWNA
PRINCETON
10,000 FT
7000 FT
5000 FT
3000 FT
PENTICTON
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
Map 4-18 - Southwest Interior
The Southwest Interior, better known as the Thompson-Okanagan, is essentially
a mountainous area bounded by the Fraser River, on the west, and the Monashee
Mountains, on the east. Most of the population and airports are located in river
valleys that have a strong north to south orientation. Only Kamloops, on the northern edge of the area, lies in an east to west valley.
This area has essentially two seasons – summer and winter, with a very short spring
and fall to mark the transition from one to the other. The climate of this area is
strongly influenced by the Coast Mountains. When the flow aloft is southwesterly, it
is common to see Pacific frontal systems produce little or no precipitation over this
area. Only when the upper flow turns to the south does significant precipitation penetrate these valleys. The annual precipitation is almost equally split among the
months – weather systems providing the precipitation in the winter and convection
in the summer.
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CHAPTER FOUR
(a) Summer
Summers in the Southwest Interior tend to be sunny and hot as the Pacific Ridge
pushes the main storm track north of the area. Those fronts that do try to move
inland are significantly weakened by subsidence to the lee of the Coast Mountains.
The cloud cover will develop noticeable breaks and precipitation, if any, will be light.
In crossing the Coast Mountains, these fronts often lose their definition or disappear
completely.
For the most part the summer is dry and hot with afternoon temperatures reaching
into the mid-to-upper 30 degree Celsius range. Under such conditions, deep convection occurs almost every day but, on many occasions, there is insufficient moisture
available to support anything but cumulus clouds. Despite this, convective turbulence
and even the occasional dust devil can provide for a turbulent ride. At this time of
the year, the upper winds are usually light so that non-convective turbulence is at a
minimum.
One major exception to this pattern is the movement of a cold low over or near the
area. These lows tend to produce broad areas of cool, cloudy weather with frequent
shower and isolated thundershowers. While the precipitation is usually light, areas of
upslope flow will see larger amounts.
The most active thunderstorms tend to occur when some sort of front of upper
trough moves across the interior, producing a band of afternoon or evening thundershowers. These thunderstorms often persist well into the night. Over most regions,
the predominant types are air mass and nocturnal thunderstorms and, when they do
occur, they generally move along the valleys. The mountainous terrain hinders the full
development of thunderstorms so that severe weather is uncommon. When it does, it
usually consists of severe lightning, large hail and downburst winds. While thunderstorms can occur at anytime during the summer, the main period is from June to
August.
(b) Winter
Winters in the Southwest Interior tend to be a long drawn out cloudy affair because
of the frequent frontal systems that move in off the Pacific and the presence of cloud
trapped in the valleys. As a front moves inland, it will weaken due to subsidence; however, usually sufficient moisture will persist to give some precipitation. In the case of
snow, accumulations are usually light in comparison to the coast. One major concern
for meteorologists is the location of the snow level, that is, the level where falling
snow changes into rain. It is worth noting here that the snow level is usually found
500 to 1,000 feet below the freezing level. It is not uncommon, especially during late
fall and early spring, to have rain falling in the valley bottoms with snow falling just
a few hundred feet above the valley bottoms. Fortunately, freezing precipitation is
rare; however, icing in this cloud can be significant.
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Areas of high pressure, unless accompanied by cold air, tend have a reduced impact
because of the widespread valley cloud. Cold air stagnating in the bottom of valleys
causes a strong low level inversion to form, which traps any moisture from local
sources. This moisture eventually forms cloud, which, despite being only a few
thousand feet thick, fills the entire valley and can persist for weeks. Depending on the
height of the inversion, the base of the valley cloud may lower enough to bring the
airports below alternate or landing limits for prolonged periods of time. This cloud
will produce some precipitation, but only light snow, in areas where the moisture
supply has been increased. Valley cloud will only move if strong winds develop in the
valley, or it will dissipate if the major moisture sources freeze over. Since the larger
lakes in the Southern Interior of British Columbia freeze over completely only in
the coldest winters, most southern valleys remain susceptible to the development of
valley cloud from November to mid-February.
Valley cloud can offer a significant icing threat. With temperatures that are
relatively warm, they contain a significant percentage of supercooled water droplets
resulting in SLD icing. In addition, these larger droplets have been known to settle
out of the cloud, producing freezing drizzle just below the cloud base.
the Yukon and the northern end of the Mackenzie River valley. This cold arctic air
moves southeastward into the Prairies but can also spread over northern and central
British Columbia. Most often, the Arctic air pushes southward into the Central
Interior before coming to rest. At the same time, Arctic air also flows through the
mountain passes from Alberta and fills the Rocky Mountain Trench. Depending on
the strength of the arctic front, winds can shift abruptly into the northwest with the
passage of the front and be gusty for several hours. At least once or twice each year,
the advance of Arctic air is so strong in British Columbia that it spreads into the
Southwest Interior.
Arctic air over the interior offers little problems other than the temperature. Most
of the valley cloud dissipates, giving clear cold days and nights. Over the bodies
of water that have remained unfrozen, such as Okanagan Lake, sea smoke will form
over the water and lift to form cumulus clouds. These clouds, if there is a significant
difference between the air temperature and the water temperature, will be turbulent,
contain icing and produce local heavy snow showers. Other than this, good flying
conditions will prevail except for some localized problems with stratus.
The upper flow during the winter months is usually a strong southwesterly. This,
combining with the local mountains, produces light to moderate, occasionally severe
mechanical and lee turbulence across the area. At the same time, the combination of
the upper flow and the channelled winds in the valleys will produce wind shears near
the top of the valley.
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CHAPTER FOUR
(c) Local Effects
COLD BRIDGE
Routes Inland from the Coast
CLINTON
PAVILION
DEADMAN
CREEK
LILLOOET
CACHE CREEK
PEMBERTON
KAMLOOPS
LAKE
ASHCROFT
LILLOOET
ET
LAKE
SPENCES
BRIDGE
KAMLOOPS
LYTTON
MERRITT
NICOLA LAKE
BOSTON BAR
10,000 FT
7000 FT
5000 FT
3000 FT
HARRISON
LAKE
2000 FT
SLY
1500 FT
TOLL BOOTH
1000 FT
600 FT
300 FT
SLY
0 SEA LEVEL
COQUIHALLA HIGHWAY
ROUTE
CHILLIWACK
HOPE
COMMON FLIGHT
ROUTES
PRINCETON
ALLISON
PASS
PENTICTON
Map 4-19 - Routes Inland from the Coast
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Hope – Lytton – Cache Creek
This route follows the Fraser Canyon from Hope to Lytton and then turns northeastward up the Thompson River Valley past Spences Bridge and Ashcroft to Cache
Creek. This has long been one of the primary routes from the coast to the interior of
British Columbia.
If the Hope area is impassable, then the Fraser Canyon as far north as Boston Bar
will also likely be impassable. During the fall and winter, the route from Lytton south
is prone to patchy fog developing overnight, which is slow to dissipate the following
day. The lowest conditions usually occur around Yale and improve further north.
Despite frequent reports of strong winds at Lytton, turbulence is seldom encountered south of Boston Bar. However, during times of strong southerly winds (30 knots
or greater) severe turbulence should be expected near Lady Franklin Rock (across
from the Yale Tunnel). North of Boston Bar, pilots have, despite wind gusts to 60
knots, noticed little turbulence. The area around the Lytton airport can be turbulent
at times due to the splitting of the strong winds into the Fraser and Thompson River
valleys.
The area to the north of Lytton is extremely dry. Occasional periods of low ceilings
and visibility due to system weather can occur year round but, for the most part, the
area offers good flying. Like the southern part of the Fraser Canyon, strong winds will
occur in the summer giving localized areas of turbulence.
Hope - Princeton
This route climbs eastward from Hope over the Coastal Mountains at the Allison
Pass in Manning Park. The route west of the Allison Pass is part of a very wet upslope regime, and is an area known to be slow to dry and become clear of low cloud
after precipitation.
Simply put, if Hope is impassable then the Hope-Princeton west of Allison
Summit is likely impassable. The Hope Slide area is particularly treacherous as the
barren hillside reflects light, especially when snow-covered, causing a “brightening” in
the cloud, making pilots think that conditions are improving ahead. Visibility falls
rapidly along this route once steady precipitation begins. As little as ten minutes can
see the route change from passable to closed.
When travelling eastbound from Hope to Princeton, experts recommend that aircraft pass Hope with sufficient altitude to safely cross Allison Pass. The low cloud
tends to lie to the east of the Skagit Valley, but elsewhere it is areas of reduced visibility that remain a problem. Also, east of the Skagit River, the valley rises very steeply
to the summit. This gradient exceeds the climb capability of most conventional aircraft, and the rapidly narrowing valley makes safe turning dangerous or impossible.
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Hope - Kamloops via the Coquihalla
This route follows Highway 5 up the narrow Coquihalla River Valley to a pass near
the Toll Booth Plaza. It carries on to the Nicola Valley at Merritt and then climbs
over a highpoint at Lac La Jeune before descending into the Thompson River valley
at Kamloops. The route from Hope to the Toll Booth Plaza, especially the section
from Portia north, is subject to frequent low ceilings and visibility in snow during the
winter. Conditions are described as being similar in characteristic to the weather
around Hope, but it occurs over a longer run. Precipitation results in a very rapid lowering of ceilings and visibility. Snow showers can result in an instant lowering of the
visibility to near zero.
North of the TollBooth Plaza following the highway is generally feasible, but low
cloud can build up in the higher ground around Lac La Jeune. A good alternate is to
use the Nicola Valley, but be aware that you can run into low cloud over the hills just
to the south of Kamloops.
For pilots flying south, even when the northern highpoint is open the Coquihalla
Pass and the area south may be closed. Also it is necessary to have scattered conditions or ceilings above 3,000 feet in the Fraser Valley or pilots can find themselves
trapped on top.
Low level turbulence is a factor mostly in the southern section of the route from
the Toll Booth to Hope where the valleys narrow and the terrain is more extreme. Lee
wave turbulence can be encountered near Merritt below 14,000 feet ASL. Also when
strong southerly winds aloft are occurring, expect moderate occasional severe turbulence at 6,000 to 7,000 feet near Yahk Mountain and Zopkios Ridge.
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113
Pemberton, Highway 97 – Lillooet – Cache Creek
GOLDBRIDGE
CLINTON
10,000 FT
7000 FT
5000 FT
3000 FT
2000 FT
COMMON FLIGHT
ROUTES
PAVILION
1500 FT
1000 FT
600 FT
LILLOOET
300 FT
0 SEA LEVEL
CACHE CREEK
PEMBERTON
ASHCROFT
LILLOOET
LAKE
Map 4-20 - Pemberton, Highway 97 – Lillooet – Cache Creek
KAMLOOPS
This more northerly route is a popular route to fly as it often avoids the poor conLYTTON
ditions and strong winds occurring in the Fraser Canyon.
However, low cloud is common near Pavilion. Occasionally in summer, strong thunderstorms can be encountered, and in fall fog can be a problem near the lakes.
MERRITT
TOLL BOOTH
COQUIHALLA HIGHWAY
HOPE
CHILLIWACK
PRINCETON
PENTICTON
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CHAPTER FOUR
COLD BRIDGE
CLINTON
Cache Creek to Kamloops
LILLOOET
PAVILION
DEADMAN
CREEK
SLY
COMMON FLIGHT
ROUTES
CACHE CREEK
PEMBERTON
ASHCROFT
10,000 FT
KAMLOOPS
LAKE
7000 FT
LILLOOET
LAKE
5000 FT
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
SPENCES
BRIDGE
SLY
KAMLOOPS
300 FT
0 SEA LEVEL
LYTTON
SLY
MERRITT
Map 4-21 - Cache Creek to Kamloops
This route follows the Thompson River valley westward via Kamloops Lake to
Cache Creek. Precipitation generally brings low cloud all along the route. Low cloud
is slow to dissipate over Kamloops Lake just west of the Kamloops Airport. Kamloops
wind strength and direction give an indication as to which end of the lake the cloud
will be heavier. If passage is difficult, beware of the sucker hole at the west end of
Kamloops Lake. Deadman CreekTOLL
headsBOOTH
north and usually looks good, but it narrows
rapidly and rises into the plateau where ceilings will be quite low. Further west tends
to be drier. Remember that the Kamloops Valley, like other valleys in the region, is
COQUIHALLA
susceptible to valley inversion
conditions. HIGHWAY
HOPE
CHILLIWACK
PRINCETON
PENTICTON
KEREMEOS
LYTTON
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MERRITT
Princeton to Penticton
TOLL BOOTH
10,000 FT
7000 FT
COQUIHALLA HIGHWAY
5000 FT
3000 FT
HOPE
2000 FT
CHILLIWACK
1500 FT
1000 FT
600 FT
PRINCETON
300 FT
0 SEA LEVEL
ALLISON
PASS
COPPER
MOUNTAIN
COMMON FLIGHT
ROUTES
PENTICTON
KEREMEOS
Map 4-22 - Princeton to Penticton
This route follows the Similkameen River Valley from Princeton to Keremeos, then
turns northeast and climbs up the Ollala Valley and over the hills before dropping
into the Okanagan Valley. Peaks that are from 3,000 to 4,500 feet higher than the valley floor encircle Princeton. The bowl thus created is often filled with fog during the
fall and spring. Fog is not so prevalent during the winter, but low valley-type cloud is
often present. The approach into Princeton is quite narrow from the south and blowing snow coming off the ridges can at times reduce visibility significantly. When the
surface winds are strong the area around Keremeos is prone to mechanical turbulence
at lower levels.
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Okanagan Valley - Kamloops - Salmon Arm
KAMLOOPS
SHUSWAP LAKE
SALMON ARM
10,000 FT
7000 FT
5000 FT
3000 FT
2000 FT
1500 FT
FALKLAND
1000 FT
MABEL LAKE
600 FT
300 FT
0 SEA LEVEL
COMMON FLIGHT
ROUTES
VERNON
KALAMALKA
LAKE
KELOWNA
SW
PENTICTON
SW
Map 4-23 - Kamloops to Salmon Arm
The Okanagan Valley extends
in a generally north-south orientation from just
BRIDESVILLE
south
of
the
Shuswap
Lake
to
the
very south of Washington State. It is one of the
OSOYOOS
driest areas in BC with desert conditions south of Penticton. The valley tends to have
steep sides with peaks along both sides from 6 to 7 thousand feet. In general, the
weather is excellent except when major systems are moving through the area.
GRAND
Winds and temperatures aloft have a strong
effect FORKS
on flying conditions in the valley.
Westerly to southwesterly winds aloft is common, and when strong, cause lee wave
TRAIL
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turbulence and downdrafts on the west side of the valley. This is aggravated in the
summer by a significant loss of aircraft performance when aircraft fly from the cooler air over the lake to the hotter air over the hills (density altitude). It can be very hazardous to try to turn and climb west or southwestward out of Penticton. Particularly
in hot weather pilots new to the area should be warned to gain sufficient altitude
southbound over the valley before turning westward enroute.
Another effect common to many BC valleys is that of temperature inversion. Often
associated with ridges, warmer air aloft traps cooler air in the valley. In the Okanagan
the lakes provide a moisture source and radiation cooling overnight aggravates the
inversion. Low cloud forms and rises as the day warms up but is capped by the inversion causing ceilings from 1,800 to 3,000 feet. This effectively limits VFR flying to
the valley itself. The longer these conditions last the longer the cloud persists each
day. After a few days it can often stay overcast until mid afternoon.
Though it is possible to fly from Kamloops into the Okanagan Valley without
climbing out of the valleys, it is a much longer route to do so. It would involve following the South Thompson to Shuswap Lake and then turning south into the
Okanagan Valley via Enderby and Armstrong. The more common and much shorter
route involves climbing out of the South Thompson valley east of Kamloops at
Monte Creek and following the highway through the valley passes of Westwold and
Falkland. The area is generally dry but after precipitation low cloud will often remain
in the upper valley passes longer than at Kamloops (the only local METAR). With
strong winds at 3 and 6 thousand feet expect mechanical turbulence in the passes. It
can also be quite turbulent at times in the wide part of the valley just east of
Kamloops.
The Sushwap Lake area is a junction for pilots transitioning from the east-west
routes to and from Alberta via Kamloops and Golden and the north-south Okanagan
route. The base of the valley cloud that extends out of the Okanagan and across the
Shuswap area is usually at the same height, except for an area around
Enderby/Armstrong and southern Kalamalka Lake, where ceilings can be considerably lower. As you approach Shuswap Lake, low cloud develops as the moisture
increases. This is quite common from October to mid-March. It is also worth
noting that the arms of Shuswap Lake freeze over but the deep central part usually
remains open.
The airport at Salmon Arm is 600 feet higher than Shuswap Lake and can make
approaches difficult during times of prolonged low valley cloud.
During the summer months, thunderstorms tend to intensify near Shuswap Lake,
and many feel that the lightning capital of BC is the area just east of the lake.
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REVELSTOKE
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GAP
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CHAPTER FOUR
Kootenays and Columbias
10,000 FT
7000 FT
VALLEY CLOUD
5000 FT
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
FAIRMONT
NELSON
300 FT
0 SEA LEVEL
CRAWFORD BAY
CASTLEGAR
CRANBROOK
CRESTON
Map 4-24 - Kootenays and Columbias
The Kootenays and Columbias consist of the eastern and southeastern sections of the
southern half of the province. Being mountainous, with most of the valleys running in
a north to south direction, the weather of this area is essentially a wetter, colder version
of the Southwest Interior. It is wetter because of the terrain that gradually increases
in height from the west until it reaches the crest of the Rockies. Any system that
crosses the Coast Mountains on an eastward track will undergo further upslope lift in
this area, resulting in additional precipitation. It is colder because of the Arctic air that
can seep through the mountain passes from Alberta.
(a) Summer
The weather during the summer is essentially benign. Frontal systems that do move
into the Southern Interior and continue eastward will begin to respond to the effects
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of upslope lift. As it does, the cloud band will thicken and the precipitation becomes
heavier and more widespread along the upslope areas. For the most part, the rainfall
is persistent but generally light. However, when a cold low moves across the area local
accumulations may reach 50 to 80 millimetres.
The majority of precipitation over this area during the summer is convective. For
the most part, the mountainous terrain hinders the development of severe thunderstorms. Instead, airmass and nocturnal thunderstorms develop and move along the
valleys. Severe convective weather is most often the result of storms moving northeastward out of Washington and Idaho. Even these storms usually produce severe
lightning, large hail and downburst winds. Only one tornado, near Cranbrook in
2001, has ever been verified. While thunderstorms can occur at anytime during the
spring and summer, the main period is from June to August.
(b) Winter
The most difficult weather in this area occurs in the winter months. Frontal
systems are stronger and wetter resulting in widespread cloudiness and precipitation.
For the most part, the precipitation will fall as snow resulting in extensive low ceilings and visibility. Accumulations are usually light in comparison to the coast. One
notable exception is along the Monashee and Columbia mountains, between
Revelstoke and Blue River, where upslope lift often produces significant accumulations.
Areas of high pressure have a lesser impact because of the widespread valley cloud.
Cold air stagnating in the bottom of valleys causes a strong low level inversion to form
that traps any moisture from local sources. This moisture eventually forms cloud
which, despite being only a few thousand feet thick, fills entire valley systems and can
persist for weeks. This cloud will only produce light snow in areas where the moisture
supply has been increased. Valley cloud will only move if strong winds develop
in the valley or it will dissipate if the major moisture sources freeze over. Since the
larger lakes and rivers seldom freeze over completely, most valleys remain susceptible
to the development of valley cloud from November to mid-February.
Incursions of arctic air from the north and east are fairly common during the
winter. A strong area of high pressure forms in the very cold air over Alaska, the
Yukon and the northern end of the Mackenzie River valley. This cold arctic air moves
southeastward into the Prairies but can also spread over northern and central British
Columbia. Most often, the Arctic air pushes southward into the Central Interior
before coming to rest. At the same time, the arctic air also flows through the mountain passes from Alberta and fills the Rocky Mountain Trench. Depending on the
strength of the arctic front, winds can shift abruptly into the east with the passage of
the front and be gusty for several hours. As the cold air deepens, it will gradually move
southward until the entire area is covered. Castlegar is often the last place in the
Southern Interior to feel the arctic air.
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CHAPTER FOUR
Arctic air offers little problems other than the temperature. Most of the valley cloud
dissipates giving clear, cold days and nights. Over the bodies of water that have
remained unfrozen, sea smoke will form over the water and lift to form cumulus
clouds. These clouds, if there is a significant difference between the air temperature
and the water temperature, will be turbulent, contain significant icing and produce
local snow showers. Other than this, good flying conditions will prevail except for
some localized problems with stratus.
(c) Local Effects
Aircraft movement in British Columbia, unless the weather is very good (clear or
high ceilings), tends to be a matter of flying established routes. There is a major route
to the north, (Salmon Arm to Revelstoke and Golden), and a southern route from
Osoyoos to Cranbrook and eastwards. Aircraft usually change from one to the other
using the Okanagan Valley and Rocky Mountain Trench. Finally, there is a well established route northwards along the Rocky Mountain Trench into the northern interior.
Southern Route – Osoyoos to Cranbrook and Eastwards
PENTICTON
FAIRMONT
OSOYOOS
NELSON
GRAND FORKS
CASTLEGAR
10,000 FT
7000 FT
TRAIL
CRANBROOK
5000 FT
3000 FT
CRESTON
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
Map 4-25 - Southern Route – Osoyoos to Cranbrook and Eastwards
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121
Osoyoos to Castlegar
PENTICTON
OSOYOOS
OSOYOOS LAKE
NELSON
10,000 FT
BRIDESVILLE
CRAWFORD
7000 FT
GRAND FORKS
5000 FT
CASTLEGAR
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
300 FT
COMMON FLIGHT
ROUTES
CHRISTINA
LAKE
TRAIL
0 SEA LEVEL
Map 4-26 - Osoyoos to Castlegar
This route climbs out of the Okanagan Valley in a steep climb to the east from
Osoyoos near Anarchist Mountain. The route has to clear the Bridesville Pass (5,000
feet ASL) to get to the Kettle Valley from Rock Creek to Midway. It then climbs
again to almost 3,500 feet before dropping into the valley at Grand Forks. From there
it follows Highway 3 past Christina Lake, rising another 2,000 feet through yet
another upslope region with the usual increase in precipitation and cloud, before
dropping into the Columbia River valley west of the Arrow Dam and Castlegar.
Another option from Grand Forks, is to fly just south of the US border eastward to
Northport, Washington and then north up the Columbia River to Trail and
Castlegar. This is much drier and lower than the more northerly route.
This route is seldom a problem during the peak of summer except for thunderstorm
activity. The area is hot and dry with little weather other than thunderstorms. These
thunderstorms usually originate in Washington State and move northwards along the
valleys during the afternoon and evening. Most dissipate by midnight but, on occasion, a few nocturnal thunderstorms will last into the early morning.
The rest of the year is another story. The combination of a narrow pass near
Bridesville and low cloud can make this route treacherous. Upslope lift east of the
Okanagan Valley and again just west of Grand Forks causes increased precipitation
and cloud and, when applicable, aggravates convective build-ups. The high ground
between Osoyoos and Grand Forks is a known snowbelt that extends in a north to
CRESTON
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south line along the peaks. Generally, when conditions are marginal in Penticton, they
are, or soon will be, worse at these points. The route from the Lower Arrow Lake to
Christina Lake is very narrow and can also be blocked by low cloud. Movement
through here is a matter of waiting for the openings. The more southerly route
through Northport, is often a more viable option.
Once into the Columbia River Valley the main problem is the Castlegar area itself.
The Arrow Dam where the Columbia River valley narrows is a known area of turbulence in strong winds. There are also the pulp mill, 5 miles west of town and the
Cominco Smelter, 10 miles south at Trail, which add condensation nuclei, aggravating the fog and low cloud problems in and around Castlegar. Usually, west of the
Arrow Dam, ceilings and visibility improve.
The valley is very deep, with Castlegar below 2,000 feet ASL and peaks all around
within 3 miles topped above 8,000 feet ASL. This combination makes Castlegar a
prime candidate for valley inversion problems when the right conditions occur, and is
also one of the last valleys to “scour out” the cold air after an arctic outbreak.
Fortunately being so far south, arctic outbreaks don’t commonly reach Castlegar.
Commercial traffic face special problems with Castlegar due to all the preceding
factors. The terrain features mean that the valley can be open to VFR flight while
closed to IFR. The approach minima range from around 3,000 feet for the GPS to
the more common LOC/DME at about 3,400 feet. As ceilings often hover in this
range and change rapidly, commercial pilots often refer to the airport as “Cancelgar”.
Being in the middle of an upslope area the Castlegar region is also susceptible to
heavy icing, especially in the mid ranges (10,000 to 16,000 feet) thus affecting mostly inbound and outbound traffic or enroute traffic without oxygen equipment.
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FAIRMONT
Castlegar to Cranbrook
10,000 FT
7000 FT
5000 FT
3000 FT
NELSON
2000 FT
CRAWFORD BAY
1500 FT
GREY CREEK PASS
1000 FT
600 FT
300 FT
CASTLEGAR
0 SEA LEVEL
TRAIL
CRANBROOK
SPARWOOD
SALMO
KOOTENAY
PASS
MOYIE
LAKE
COMMON FLIGHT
ROUTES
FERNIE
CRESTON
YAHK
RYAN
Map 4-27 - Castlegar to Cranbrook
There are several routes that are used, depending on the weather of that particular
day. The route from Castlegar to Creston along Highway 3 requires passing through
a high mountain pass between Salmo and Creston, which makes this an unpopular
route with most pilots. Weather along this route has forced a few emergency landings
on the highway. In addition to low cloud, strong winds with wind shear are common.
A second possible route from Castlegar to Creston is to go east past Nelson until
you reach Crawford Bay and then turn south along Kootenay Lake. With all mountain routes clouds and winds can be a problem. Low-level winds can be strong in the
Kootenay Bay area due to converging winds where the valleys come together. Winds
here and at Nelson tend to be worse in a southwest flow. Nelson airport itself is
known to have crosswinds on a regular basis. In general, weather is reported to be
better east of Nelson than between Nelson and Castlegar. In fact, pilots have reported
some of the worst icing ever seen north of the Castlegar Airport, over the
Brilliant Beacon. This icing is typically between 10,000 and 16,000 feet ASL.
The trip from Creston to Cranbrook has its own problems. The Moyie River Valley
between Yahk and Moyie Lake receives more precipitation than other areas, even during the summer. The corner near Yahk, on occasion, is turbulent and can distract
pilots into taking the southern valley into the United States by mistake. There are two
areas where low cloud tends to linger. These are between Moyie and Ryan and near
ELKO
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the junctions of Hwy. 3 and 95 and extending 5 miles to the west.
The third option involves going directly between Crawford Bay and Cranbrook,
and this is the route preferred by many pilots travelling between Castlegar and
Cranbrook. The highest point is Grey Creek Pass, approximately 5 miles east of
Crawford Bay. In the pass itself, it is recommended that you follow the road rather
than the power line. When travelling west to east after crossing the pass, the weathFAIRMONT
er tends to improve along the rest
of the route. To get through the pass, ceilings must
be higher than 7,500 feet ASL. If the route closes behind you, there is no way out;
you are committed to going east into the Trench. When travelling east to west, route
finding is more difficult due to the many side valleys.
Cranbrook and eastwards through the Crowsnest Pass
NELSON
10,000 FT
7000 FT
5000 FT
STLEGAR
3000 FT
2000 FT
1500 FT
1000 FT
CRANBROOK
SPARWOOD
600 FT
BULL
RIVER
SALMO
MOYIE
LAKE
300 FT
0 SEA LEVEL
FERNIE
CRESTON
YAHK
RYAN
COMMON FLIGHT
ROUTES
ELKO
Map 4-28 - Cranbrook and eastwards through the Crowsnest Pass
At the Cranbrook airport itself, although winds are not very strong, the prevailing
westerly wind is across the runway. Shear has also been reported as significant within 200 feet above the runway The route from Cranbrook follows the Rocky Mountain
Trench south to the Elk River Valley, then turns northeast up the valley to Fernie,
Sparwood and east. The weather at Cranbrook Airport tends to be representative of
the weather along the Trench. However, low cloud is more common over the
Kootenay River. If the weather is bad at the airport, the Trench is more than likely
closed.
The route up the Elk Valley to Sparwood and on to Blairmore (in Alberta) is well
known for its strong winds (up to 80 knots) and turbulence. Planes have been forced
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down near Blairmore by severe turbulence. Local pilots say, “A nice day in Blairmore
is like winning the lottery.” On most days, the north side of the valley is less turbulent than the south side. Drier weather is experienced along the Iron Creek from
Fernie to Bull River. The pass between Fernie and Bull River is easy to see from just
west of Fernie. If this pass is closed, then similar conditions likely exist along the
highway to the south of Fernie. A heavy snowbelt runs along the Rockies from the
Bull River to the Flathead River. Strong outflow winds are common out of the Bull
River valley.
Northern Route – Salmon Arm to Banff
10,000 FT
7000 FT
5000 FT
3000 FT
DONALD
2000 FT
1500 FT
SHUSWAP LAKE
ROGERS PASS
1000 FT
600 FT
300 FT
REVELSTOKE
SALMON ARM
0 SEA LEVEL
GOLDEN
BANFF
3 VALLEY GAP
FALKLAND
Map 4-29 - Northern Route – Salmon Arm to Banff
BLAEBERRY PASS
Salmon Arm to Revelstoke
COMMON FLIGHT
ROUTES
DONALD
SHUSWAP LAKE
EAGLE PASS
GOLDON
REVELSTOKE
SALMON ARM
3 VALLEY GAP
10,000 FT
7000 FT
5000 FT
3000 FT
FALKLAND
2000 FT
1500 FT
MABEL LAKE
1000 FT
600 FT
300 FT
0 SEA LEVEL
Map 4-30VERNON
- Salmon Arm to Revelstoke
This route follows the Eagle River east through the Eagle Pass and then cuts
through a north-south range of mountains at Three Valley Gap (note that these are
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narrow gorges rather than high passes). At Three Valley Gap, the narrow confluence
of valleys channels both wind and low cloud and is the usual problem point when
conditions are marginal. When winds are strong pilots should assume that there will
be significant mechanical turbulence in this area. The upslope lift encountered by
weather system approaching from the southwest or west increases the instability and
precipitation. This causes ceilings and visibility to be lower in the vicinity of Three
Valley Gap than in Kamloops or Salmon Arm. If conditions are low in Salmon Arm,
there is a strong chance you won’t be able get past this chokepoint. However, when
going the other way, if you can get out of Revelstoke, you can usually get to Salmon Arm.
Revelstoke sits in a deep valley with the airport beside a lake. The peaks in the
vicinity rise from 8,500 to 10,000 feet above the valley floor. The Revelstoke area,
including the valleys immediately to the east and west on this route, is very susceptible to low cloud and fog, especially in the mornings. The most common low visibility is in the flats near the airport. Some mornings, the entire region can be clear while
fog closes both approaches to the Revelstoke Airport. Surface winds at Revelstoke
can be very strong following an arctic front but, in general, are not usually a hazard to
aviation. Note that, as is often the case in BC valleys, when combined with a sufficiently strong valley inversion, the low cloud can persist for much of the day.
At Revelstoke, it is possible to continue eastwards towards Golden or to turn
northwards towards the Mica Dam. Of the two, the eastward route is the preferred.
Mostly helicopters use the route from Revelstoke north to Mica Dam. The route is
essentially two 700-foot steps with ceilings dropping as one travels north. Pilots must
be aware that low cloud is common over open water, and beware of the power lines
crossing the route several times north of Downie Creek.
BLAEBERRY PASS
Revelstoke to Golden to Banff
COMMON FLIGHT
ROUTES
10,000 FT
DONALD
7000 FT
5000 FT
3000 FT
SHUSWAP LAKE
2000 FT
KATABATIC WINDS
REVELSTOKE
M
N
11:30 PM
ROGERS PASS
GOLDEN
3 VALLEY GAP
MABEL LAKE
Map 4-31 - Revelstoke to Golden to Banff
KICKING HORSE
PASS
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
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This part of the route climbs eastward through a highpoint at the Rogers Pass. It
then drops into the Columbia River valley and Golden. From Golden it follows the
Kicking Horse River to Field then climbs over the mountains at the Kicking Horse
Pass to Banff in Alberta.
Although a relatively wide mountain pass, the Rogers Pass (elevation 4,534 feet
ASL) has intimidated even experienced pilots. Low cloud tends to pile up west of the
summit of Rogers Pass making passage difficult. The weather often looks good entering Rogers Pass from the east; however, getting to Rogers Pass summit does not
assure passage to Revelstoke. Generally speaking ceilings at Revelstoke and Golden
need to be at least 3,000 to 4,000 feet AGL for the likelihood of the pass being flyable. It is rarely open during times of precipitation and should not be attempted when
conditions are deteriorating rapidly from the west (i.e. a front moving in).
The route from Golden to Banff crosses a major hydrological regime change over
the Continental Divide. Generally from Lake Louise east is relatively drier.
Differences in aviation weather along this route can be equally as dramatic. While
winds are generally not a major concern between Golden and Kicking Horse Pass,
local funnelled winds can provide some turbulence. The pass itself, at 5,350 feet ASL,
is dominated on either side by peaks to over 11,000 feet. When the winds are strong,
the worst turbulence, often moderate or greater, is usually found to the east of Banff,
and can make an otherwise clear day unflyable. Even higher altitudes, say 10,000 to
16,000 feet ASL, often provide no refuge from the turbulence due to the frequency
of lee wave activity. A pressure difference on either side of the Rockies usually indicates winds and associated turbulence in the pass and vicinity.
Photo 4-2 - Kicking Horse Pass
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CHAPTER FOUR
Rocky Mountain Trench
GOLDEN
BANFF
10,000 FT
7000 FT
5000 FT
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
FAIRMONT
CRANBROOK
Map 4-32 - Rocky Mountain Trench
The southern portion of the Rocky Mountain Trench follows the Columbia River
Valley northwest from the U.S. border past Cranbrook. From here the “Trench” continues past Golden to Kinbasket Lake (created by the Mica Dam). North of
Kinbasket Lake, the Rocky Mountain Trench, holding true to its course, continues
through Valemount, Mcbride and Mackenzie. From Mackenzie it forms the valley of
the Williston Lake and continues northwestward eventually reaching Watson Lake
and the Yukon.
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Cranbrook to Golden
BANFF
GOLDEN
10,000 FT
7000 FT
5000 FT
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
COMMON FLIGHT
ROUTES
INVERMERE
COLUMBIA
RIVER
FAIRMONT
COLUMBIA LAKE
CANAL FLATS
CRANBROOK
Map 4-33 - Cranbrook to Golden
KOOTENAY
RIVER
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CHAPTER FOUR
This route follows the Columbia River valley northwest up the Rocky Mountain
Trench to Golden. With the Purcell Mountains to the west and the Rocky
Mountains to the east, this route may receive the best aviation weather in all of
Western Canada. System weather tends to be lighter in the south and is generally
weakened by subsidence. Weather-related hazards are few except for relatively frequent low cloud and fog near Parson. Radiation fog is common under clear skies in
autumn although it generally burns off by 10 a.m. In winter, valley cloud between
4,000 and 6,000 feet ASL (ceilings from 1,200 to 2,000 feet) often closes higher
routes and produces significant icing. In general when cloudy conditions do prevail,
aircraft can encounter heavy icing conditions in the cloud east of Cranbrook and
Golden in the upslope conditions of the western slopes of the Rocky Mountains.
Infrequent strong winds do not usually produce significant turbulence due to the
wide, linear valley, except where valleys merge with the Trench from the side. With
strong winds aloft (west to southwest), occasionally lee wave turbulence will affect
flights into and out of Cranbrook.
Golden Airport is not usually representative of the weather along the Trench. It
tends to be much drier than areas to the north and south, especially the north. The
airport at Cranbrook is on a hill above the town and is often subject to crosswinds
from the west. Note that if the weather is bad at Cranbrook it is likely as bad or worse
in the Trench between Golden and Cranbrook.
Blaeberry Pass, Golden to the North Saskatchewan Crossing
BLAEBERRY
PASS
10,000 FT
7000 FT
5000 FT
3000 FT
2000 FT
DONALD
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
KATABATIC
COMMON FLIGHT
ROUTES
FIELD
GOLDEN
BANFF
Map 4-34 - Blaeberry Pass, Golden to the North Saskatchewan Crossing
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Another common route out of the Rocky Mountain Trench is Blaeberry Pass along
the Blaeberry and Howse Rivers. Opening out of the Rocky Mountain Trench just
north of Golden the Blaeberry Valley climbs to the Blaeberry Pass. The valley
becomes very narrow and deep as it approaches the pass and has peaks on either side
from 8 to 11 thousand feet high. East of the Blaeberry Pass the valley follows the
Howse and then the North Saskatchewan Rivers. Beside the usual cloud problems
associated with Rocky Mountain passes, this route has the potential for very hazardous wind and turbulence conditions. An extremely bad spot, even for helicopters,
is at Mummery Creek. West of this location, winds and turbulence diminish rapidly
except for extreme katabatic winds on sunny summer evenings. With the effects of
channelling and rugged terrain on valley winds and the katabatic winds off the ice
fields and glaciers, wind patterns can be very chaotic. The only way to avoid these
conditions is to fly before 10am or after 5pm when surface winds are light or at much
higher altitudes when ceilings allow it. Clearly this is not a recommended route for
light, low performance aircraft if low level valley flying is involved.
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CHAPTER FOUR
Golden - the Mica Dam – Tête Juane Cache - Jasper
TÊTE
TE JAUNE CACHE
VALEMOUNT
SW
JASPER
10,000 FT
7000 FT
5000 FT
3000 FT
2000 FT
1500 FT
1000 FT
SW
VALLEY CLOUD
600 FT
300 FT
0 SEA LEVEL
SW
KINBASKET
LAKE
COMMON FLIGHT
ROUTES
MICA
CREEK
DOWNIE
CREEK
DONALD
ROGERS PASS
SHUSWAP LAKE
REVELSTOKE
GOLDEN
3 VALLEY GAP
Map 4-35 - Golden - the Mica Dam – Tête Juane Cache - Jasper
BANFF
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This route follows the Rocky Mountain Trench northwest to Tete Jaune Cache
where is turns sharply east past Mount Robson and up the Fraser River headwaters
to the Yellowhead Pass into Alberta. Although still in the relatively dry Rocky
Mountain Trench, this section receives considerably worse weather than areas further
south. Just north of Golden the main route along Blackwater Creek is higher than an
alternate route which stays over the river and goes around Blackwater Mountain;
however, in bad weather, experienced pilots recommend neither route. Winds along
the Columbia River are almost always much stronger than those further south at
Golden. The water usually freezes by mid winter, reducing the amount of fog and low
cloud.
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CHAPTER FOUR
Central and Northern Interior
CARCROSS
SKAGWAY
TESLIN
10,000 FT
ATLIN
7000 FT
PINE LAKE RANCHERIA
5000 FT
3000 FT
WATSON
LAKE
2000 FT
1500 FT
1000 FT
600 FT
DEASE
LAKE
TELEGRAPH
CREEK
300 FT
LIARD
RIVER
STIKINE
0 SEA LEVEL
ISKUT
TOAD RIVER
SUMMIT LAKE
STEAM BOAT
BOB
QUIN
STEWART
SIFTON
PASS
FORT NELSON
FORT
WARE
MEZIADIN
PROPHET
RIVER
INGENIKA
SIKANNI
CHIEF
NEW HAZELTON
TERRACE
SMITHERS
FORT ST. JOHN
BURNS
LAKE
MACKENZIE
PINE
PASS
CHETWYND
DAWSON CREEK
FORT ST. JOHN
TUMBLER
RIDGE
VANDERHOOF
PRINCE
GEORGE
BELLA COOLA
GOATRIVER
ANAHIM LAKE
QUESNEL
MCBRIDE
ALEXIS CREEK
PUNTZI
MOUNTAIN
QUESNEL LAKE
TÊTE JAUNE CACHE
WILLIAMS LAKE
VALEMOUNT
JASPER
BLUE RIVER
CLINTON
PAVILION
LILLOOET
PEMBERTON
KAMLOOPS
SALMON ARM
Map 4-36 - Central and Northern Interior
GOLDEN
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The Central and Northern Interior occupies the bulk of the BC Interior. The
Central Interior is essentially the area from just north of an east to west line through
Clinton to a rough east-west line through Mackenzie. The Northern Interior extends
from that point to the Yukon border. Bounded to the west by the Coastal Mountains
and to the east by the Rockies, the area has an extremely wide variation in climate
between summer and winter. Summers tend to be pleasant while winters can range
from seasonably cold to outright frigid.
(a) Summer
Like the rest of BC, summer is the benign season where the weather is strongly
influenced by the position of the main storm track. During the early part of summer
and in the fall, the storm track tends to lie over the Central Interior. This exposes this
area to the travelling frontal systems sweeping across the area giving frequent cloudiness and precipitation.
In the middle of summer, low pressure areas usually remain offshore as the Pacific
High strengthens and moves further north. This northward shift causes the main
storm track to shift into the northern Gulf of Alaska and across northern British
Columbia, or even into the Yukon. South of this track, minor frontal systems, upper
troughs and thunderstorms produce most of the weather. In August and September,
weeks can pass between weather systems.
In the Central Interior of British Columbia and the northern section of British
Columbia, the plateau type terrain allows the thunderstorms to reach full intensity.
While air mass thunderstorms still remain the predominate type, frontal thunderstorms and nocturnal thunderstorms are common. The typical scenario would see
the beginning of thunderstorm activity early in the afternoon and for it to dissipate
in the evening. Most often the thunderstorms move towards the northeast and, given
the right conditions, their intensity can reach the severe level. Within BC, it is the
area around Prince George that has the potential each year to produce tornadoes. The
normal thunderstorm season for both areas is June to August.
(b) Winter
As Pacific frontal systems move inland, they do not weaken as much as they do in
the south. This is simply because the Coast Mountains are not as high at these
latitudes. As such, steady precipitation is to be expected whose type will vary with the
local temperatures. Accumulations are usually light, however, local accumulation can
be larger especially near the Arctic front.
The northern half of British Columbia is subject to widespread valley cloud only
during the early part of the season, as the lakes and rivers generally freeze over completely. Thus, ridges of high pressure during mid and late winter bring widespread
clear, cold weather.
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CHAPTER FOUR
the Yukon and the northern end of the Mackenzie River valley. This cold arctic air
moves southeastward into the Prairies but can also spread over northern and central
British Columbia. Most often, the arctic air pushes southward into the Central
Interior before coming to rest somewhere near Clinton. At the same time, arctic air
also flows through the mountain passes from Alberta and fills the Rocky Mountain
Trench. Depending on the strength of the arctic front, winds can shift abruptly into
the northwest to northeast with the passage of the front and be gusty for several hours
Arctic air over the interior offers little problem, other than the temperature. Most
of the valley cloud dissipates, giving clear cold days and nights. Over the bodies of
water that have remained unfrozen, such as Williston Lake, sea smoke will form over
the water and lift to form cumulus clouds. These clouds, if there is a significant difference between
air temperature and the water temperature, will be turbulent,
ALEXISthe
CREEK
contain icing and produce local snow showers. Other than this, good conditions will
prevail
except for some localized problems with ice fog.
PUNTH
QUESNEL LAKE
MOUNTAIN
(c) Local Effects
WILLIAMS LAKE
Lillooet - Pavilion - Clinton
PLATEAU
OFTEN COVERED
IN LOW CLOUD
10,000 FT
3000 FT
2000 FT
KELLY
LAKE
100 MILE
HOUSE
COMMON FLIGHT
ROUTES
7000 FT
5000 FT
108 MILE HOUSE
CLINTON
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
PAVILION
LILLOOET
PEMBERTON
Map 4-37 - Lillooet - Pavilion - Clinton
This is the continuation of the route from the south coast via Howe Sound,
KAMLOOPS
Whistler and Pemberton. At Clinton it meets the route which
runs north from Cache
Creek to Williams Lake. From Pemberton it follows the valley along Anderson and
SALMON
Seton Lakes to Lillooet. It then follows the Fraser River to Pavilion
beforeARM
turning
northeastward to climb onto the Interior Plateau at Kelly Lake and on to Clinton.
Low cloud and fog are usually concentrated over the plateau while the lower elevations, such as along the Fraser River Valley, are often below the cloud deck. If the ceiling is too low to get through Pavilion, it is likely the same at Kelly Lake.
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CREEK
Clinton ALEXIS
to Williams
Lake
QUESNEL LAKE
COMMON FLIGHT
ROUTES
WILLIAMS LAKE
10,000 FT
7000 FT
5000 FT
108 MILE HOUSE
3000 FT
2000 FT
100 MILE
HOUSE
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
CLINTON
Map 4-38 - Clinton to Williams Lake
PAVILION
This route rises gradually on the Interior Plateau to 100 Mile House. North of 108
Mile Airport it follows a LILLOOET
valley again from Lac La Hache to Williams Lake where it
rejoins
the Fraser River.
PEMBERTON
Anywhere in the central and northern regions of the province winter weather is
greatly influenced by the position of the Arctic Front. During the winter months it
KAMLOOPS
will generally be somewhere along this route or just north.
Where the Arctic Front
combines with a moisture source either on the ground or aloft in the form of frontal
SALMON ARM
activity, expect flurries and low visibilities.
Low cloud develops in the rising ground just south of Clinton and usually extends
across the Central Plateau. Conditions along the route between 100 Mile House and
Williams Lake tend to be uniform, although ceilings lower as the terrain rises when
heading southeast. Ceilings are much lower over the higher ground while fog is more
prevalent in the valley.
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PRINCE
GEORGE
CHAPTER FOUR
Williams Lake - Alexis Creek – Puntzi Mountain – Anahim Lake
ANAHIM LAKE
CHARLOTTE LAKE
PUNTZI
MOUNTAIN
KLEENA KLEENE
TATLA LAKE
10,000 FT
QUESNEL
VERY DRY
EXECPT IN EAST
OR NORTHEAST WIND
ALEXIS CREEK
7000 FT
5000 FT
3000 FT
2000 FT
1500 FT
RISKE CREEK
1000 FT
600 FT
300 FT
CHILKO LAKE
WILLIAMS LAKE
COMMON FLIGHT
ROUTES
0 SEA LEVEL
Map 4-39 - Williams Lake - Alexis Creek – Puntzi Mountain – Anahim Lake
As an alternative route to the coast through the Coastal Mountains, this route
crosses the Interior Plateau westward from Williams Lake. Leaving Williams Lake
the route runs south before crossing the Fraser River to climb over a highpoint west
of Riske Creek at almost 4,000 feet ASL. It then weaves its way west over the high
plateau, following the Chilcotin and Chilanko Rivers just south of Puntzi Mountain
to Tatla Lake. Then turning more northwest, it crosses the plateau to Nimpo and
Anahim Lakes.
Generally, weather systems give more precipitation east of the Fraser River.
However, low cloud and fog are common to both areas in the fall and early winter
before freeze-up and again during spring melt, especially at the lower elevations. This
is also true for the section from Alexis Creek to Puntzi Mountain, especially near the
larger lakes until they freeze over. However, low cloud can occur with a moist easterly circulation at any time of year. In this more western section the fog and low cloud
are more likely to burn off by late morning.
Over the eastern section of the Interior Plateau, light to moderate turbulence usually occurs during the afternoon in the summer season due to convective heating.
Thunderstorms are more frequent, especially in the vicinity and east of the Fraser River.
West of the Fraser River, cells are generally smaller and in the developmental stage.
Lying immediately to the lee of the Coast Mountains, the region between Puntzi
Mountain and Anahim Lake is dry belt country. In the winter severe lee wave turbulence to the east of the Coast Mountains is common with very strong westerly or
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southwesterly winds aloft. This lee area includes the entire stretch of lakes from
Chilko and Tatla in the south, to Nimpo and Anahim in the north. In most cases it
is impractical to climb above this turbulence, as it would necessitate ascending to
nearly 15,000 feet ASL. Experienced pilots suggest thatVANDERHOOF
it is best to stick close to the
sides of the valleys.
Anahim Lake to the Coast
PRINCE
See data for this route as described in a previous section under
“Central Coast to
GEORGE
the Interior Plateau”.
Williams Lake to Quesnel
ANAHIM LAKE
CHARLOTTE LAKE
PUNTZI
MOUNTAIN
KLEENA KLEENE
TATLA LAKE
10,000 FT
QUESNEL
VERY DRY
EXECPT IN EAST
OR NORTHEAST WIND
ALEXIS CREEK
7000 FT
5000 FT
3000 FT
2000 FT
1500 FT
RISKE CREEK
1000 FT
600 FT
CHILKO LAKE
300 FT
WILLIAMS LAKE
COMMON FLIGHT
ROUTES
0 SEA LEVEL
Map 4-40 - Williams Lake to Quesnel
The Fraser River valley from Williams Lake to Quesnel is relatively broad and the
grade is gradual. For most of the year, aircraft operations are not hindered excessively by the weather. There is a notable exception during the winter. Whenever the arctic front oscillates back and forth through the area, the front is accompanied by freezing drizzle, low cloud, poor visibility in snow, fog and wind shear. The CaribooCentral Interior is not prone to winds. However, the Fraser River Valley can be windy,
particularly southerly winds, which can exceed 40 knots at low levels, in autumn and
winter. Although they are strong and generate high wind shear values, these winds do
not generally produce significant turbulence in the Valley.
Fog that fills the valley between Williams Lake and Quesnel is often below the
much higher altitude Williams Lake Airport. However, it must be recognized that, in
cases of extensive low cloud, the valley may be suitable for VFR flying while the
Williams Lake Airport is closed due to low ceilings.
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CHAPTER FOUR
Flight routes east into the valleys of the Cariboo Mountains are common, especially
during the summer months. Weather systems impact somewhat harder in these areas,
with lower ceilings and visibility. This area is well known for its summer thunderstorm activity with the accompanying gusty winds and turbulence.
Quesnel to Prince George
10,000 FT
7000 FT
5000 FT
OOTSA LAKE
3000 FT
VANDERHOOF
2000 FT
1500 FT
1000 FT
600 FT
PRINCE GEORGE
300 FT
0 SEA LEVEL
COMMON FLIGHT
ROUTES
HIXON
ANAHIM LAKE
QUESNEL
Map 4-41 - Quesnel to Prince George
Fog is relatively common along the valley, especially near Prince George and
Quesnel, due to the mills located
just north
of the Prince George Airport and just
ALEXIS
CREEK
south of the Quesnel Airport. Some days, the fog will extend only a few miles from
the airports with clear conditions elsewhere. The emissions from the mill stacks often
give a visual representation of the low-level vertical wind pattern. QUESNEL LAKE
WILLIAMS LAKE
Turbulence is not usually strong along this route, except near Trapping Lake up to
8,000 or 9,000 feet ASL. In summer, convective cloud is most common in a north to
south band through Hixon.
The most significant hazard around the Prince George Airport is low cloud and
fog, enhanced by the moisture along the rivers as well as the pulp mills north of the
airport. Even on a clear day, fog associated with the mills can obscure the approach
to runway 15. In this case, the southern approach to runway 33 is generally better.
Strongest winds at the airport tend to be from the south. Light northerly winds at the
surface often occur in conjunction with strong southerly winds aloft, say at 800 feet.
The loss of airspeed on takeoff can cause small aircraft to drop into the river valley,
near the bluffs on the north side of the river. This is most significant under morning
inversions.
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Kamloops –Vavenby – Blue River – Tête Jaune Cache
QUESNEL LAKE
TÊTE
TE JAUNE CASHE
CACHE
VALEMOUNT
COMMON FLIGHT
ROUTES
JASPER
BLUE RIVER
10,000 FT
CLINTON
7000 FT
5000 FT
3000 FT
2000 FT
1500 FT
CLEARWATER
VAVENBY
1000 FT
600 FT
300 FT
0 SEA LEVEL
LITTLE
FORT
BARRI RE
BARRIÈRE
ADAMS
LAKE
SW
KINBASKET
LAKE
SW
SW
KAMLOOPS
Map 4-42 - Kamloops –Vavenby – Blue River – Tête Jaune Cache
SALMON ARM
GOLDEN
Two routes are commonly used between Kamloops and Vavenby. The first follows
Highway 5 up the North Thompson River through Barrière and Clearwater to
Vavenby. The second follows the South Thompson River east from Kamloops to
Little Shuswap Lake and then turns north up the Adams Lake and River. North of
Adams Lake it traverses a narrow pass to emerge into the North Thompson River valley just east of Vavenby. Once north of Vavenby the route follows the North
Thompson River past Blue River. The long narrow valley climbs gradually to cross a
north/south divide just south of Valemount, where it turns northwest up the Rocky
Mountain Trench to Tete Jaune Cache. Here it meets the east-west route which links
north central BC with Alberta via Prince George and Jasper, basically following Hwy
16, the Yellowhead Highway.
Both routes from Kamloops to Vavenby can see some low cloud especially during
and after precipitation. Just to the north of Adams Lake, the combination of low
cloud over rising ground and narrow passes can be quite hazardous. Depending on the
moisture and the time of year, ceilings can be right down onto the lake. It should be
noted that Adams Lake remains open all year.
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CHAPTER FOUR
North of Blue River tends to be somewhat drier. It should be noted that the Blue
River weather observation does not usually report the low cloud that forms over the
rising ground just to the south. Note that mountain peaks on both sides of the valley
are between 5 and 6 thousand feet south of Blue River while those along the narrow
valley to Tete Jaune Cache, are in the range of 8 to 10 thousand feet. North of Blue
River, strong winds aloft (6 to 12 thousand feet) out of the west or southwest can
cause considerable turbulence in the valley.
Tete Jaune Cache - Mcbride - Prince George
VANDERHOOF
10,000 FT
7000 FT
5000 FT
PRINCE
GEORGE
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
GOATRIVER
COMMON FLIGHT
ROUTES
SW
QUESNEL
MCBRIDE
SW
QUESNEL LAKE
WILLIAMS LAKE
TÊTE
TE JAUNE CACHE
VALEMOUNT
BLUE RIVER
Map 4-43 - Blue River – Valemount - Tete Jaune Cache – Goat River – Prince George
The Rocky Mountain Trench creates a natural route from Tete Jaune Cache to the
northwest. The route follows the Fraser River up the Trench to a point about 30 miles
east of Prince George then turns west directly to the airport.
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Summer convective cloud is common along both sides of the Trench over the higher
ground. Severe turbulence often results when strong winds aloft cross the mountains
from the southwest. This turbulence does not generally extend above the terrain
height, 6,000 to 8,000 feet. Severe turbulence is normal on summer afternoons over
the glaciers to the northeast.
In winter low cloud and precipitation move through with frontal disturbances, drying slowly behind them. As is usual in the north, the position of the Arctic Front is
pivotal, as there are commonly flurries where there is any moisture along the front and
it tends to be colder and drier to the north of the front.
Prince George to Mackenzie
FORT ST. JOHN
PINE
PASS
CHETWYND
MACKENZIE
FORT ST. JAMES
PARSNIP
RIVER
TUMBLER
RIDGE
10,000 FT
SUMMIT LAKE
7000 FT
5000 FT
VANDERHOOF
3000 FT
2000 FT
1500 FT
PRINCE
GEORGE
Map 4-44 - Prince George to Mackenzie
COMMON
ROUTES
1000 FT
600 FT
300 FT
0 SEA LEVEL
Following Highway 97 north, this route passes Summit Lake and continues up the
Rocky Mountain Trench to McLeod Lake and Mackenzie. An alternate route leaves
Prince George and follows the Fraser River to the north-northeast and then over the
hills to enter the Parsnip River Valley. This valley runs parallel and just to the east of
the first route, merging with it at Mackenzie.
Low cloud and fog are often encountered near Summit Lake, especially in the fall
and spring. Poor conditions will at times extend as far as McLeod Lake. Winds and
turbulence are generally light along this route. Low cloud and fog are commonly
encountered along the Parsnip River Valley, especially in the fall and spring and often
in association with frontal precipitation.
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CHAPTER FOUR
Pilots will sometimes follow the Parsnip River Valley south from Mackenzie, then
east along the Fraser River into Prince George.
Crossing the Rockies: Mackenzie to Dawson Creek/Fort St. John
FORT ST. JOHN
PINE
PASS
CHETWYND
MACKENZIE
PARSNIP RIVER
DAWSON CREEK
10,000 FT
MONKMAN
PASS
7000 FT
5000 FT
TUMBLER
RIDGE
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
COMMON
ROUTES
300 FT
0 SEA LEVEL
Map 4-45 - Crossing the Rockies Mackenzie to Dawson Creek/Fort St. John
There are two main routes across the Rockies at this point. There is a southern
route via Tumbler Ridge and the more direct route via the Pine Pass and Chetwynd
following Highway 97. This route climbs the upslope side of the Rocky Mountains
to the narrow Pine Pass. It then follows the Pine River out of the mountains onto the
plains near Chetwynd. The worst section along this route is near Pine Pass where low
cloud, turbulence and very heavy snow are common. The Powder King Ski Hill near
Pine Pass receives approximately 1,200 centimetres of snow each winter. This heavy
snow occurs in a band from Pine Pass to McLeod Lake. In summer, the Pine Pass
area is also prone to thunderstorm activity. As with other locations just east of the
Rockies, lee wave turbulence is to be expected in the Chetwynd area when there is a
strong southwesterly flow aloft
Thunderstorms often form along the foothills in summer months along a line just
east of Chetwynd. These convective lines tend to remain relatively stationary while
developing in the early afternoon, then tend to begin to move in the direction of the
upper level (approximately FL180) winds late in the day.
When flying via Tumbler Ridge, most pilots choose to take the route along the
Murray River and the Monkman Pass. Between Tumbler Ridge and points southeast,
a route via the McGregor River to Monkman Pass (Monkman Lake), then along the
Murray River, is utilised. There are, however, several dangerous canyons that can be
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mistaken for the main route. When using this route, look for strong turbulence near
Monkman Pass. The onset of turbulence can be very abrupt when approaching from
the east.
The Murray River and Monkman Pass area generally experiences drier and calmer
weather than Pine Pass, throughout the year.
Turbulence in this region is worst in north to south oriented valleys, when the
winds are blowing across the mountains from west to east. These winds tend to be
strongest in spring and autumn, with associated mechanical turbulence usually below
1,000 feet AGL.
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CHAPTER FOUR
Mackenzie to Watson Lake along the Rocky Mountain Trench
WATSON LAKE
LOWER POST
LIARD
RIVER
TOAD RIVER
SUM
SIFTON PASS
FORT
WARE
INGENIKA
COMMON
ROUTES
WILLISTON LAKE
10,000 FT
7000 FT
5000 FT
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
CHE
PINE PASS
300 FT
0 SEA LEVEL
MACKENZIE
Map 4-46 - Mackenzie to Watson Lake along the Rocky Mountain Trench
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Mackenzie lies at the south end of Williston Lake, a long narrow body of water
which fills this section of the Rocky Mountain Trench. North of Williston Lake, the
terrain begins to rise and the valley narrows. The Trench continues in a near straight
line to the northwest past Fort Ware, over the Sifton Pass and on, eventually descending and widening into the basin of Watson Lake.
Mackenzie, and the high ground to the east and west of Mackenzie, is located in
one of the snow belts of the interior of BC. Smoke from the mills near the Mackenzie
Airport can induce the formation of low cloud in the area. Likewise, low cloud and
fog are prevalent near and over Williston Lake, particularly from late summer to early
winter. Between the north end of Williston Lake and Fort Ware, fog is common in
summer and autumn, especially near Finbow. This section, north of Williston Lake,
has a higher frequency of low cloud, and the Sifton Pass area, reputedly the worst spot
along the Trench, is frequently reported by pilots to be turbulent or have cloud down
to the treetops.
Lee wave activity and subsiding air currents are common along the west side of the
Trench, due to airflow off the high ground into the valley. The east side can be better, but turbulence can be severe particularly about mid way up Williston Lake,
around the entrance and into the Peace and Ospika Arms.
North of the Sifton Pass, southbound pilots must exercise caution not to mistake
the Gataga River Valley, which angles off to the southeast, for the Trench. Similarly,
northbound pilots often mistake the Kechika River for the more westerly route to
Watson Lake.
Though this is a long stretch of territory there are only weather observations available at each end: the one from the automatic weather station at Mackenzie and that
from Watson Lake.
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CHAPTER FOUR
NEW HAZELTON
Prince George to Smithers
TAKLA LAKE
SMITHERS
TELKWA
BABINE LAKE
HOUSTON
MACK
BURNS LAKE
STUART LAKE
FORT ST. JAMES
FRANCOIS LAKE
FRASER LAKE
OOTSA LAKE
10,000 FT
7000 FT
VANDERHOOF
5000 FT
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
COMMON
ROUTES
PRINCE
GEORGE
Map 4-47 - Prince George to Smithers
This route follows Highway 16 from Prince George to Vanderhoof, then on
through Fraser Lake, Burns Lake, Houston, and the town of Telkwa to Smithers.
Being east of the coastal mountain range, this route is in the generally drier inland
climate region. The lakes are at the lowest points, especially Burns Lake, which sits
in a bowl relative to the higher ground all around it. As a result cold air pools there
and it is always one of the coldest spots in the region. It freezes over early in the winter and is subject to fog.
This route is fairly dry in the summer, especially between Burns Lake and
Smithers. That being said, some of the strongest convection in the BC Interior can
be found between Prince George and Burns Lake. Large thunderstorms with strong
winds and hail occur, with the occasional report of a funnel cloud or tornado.
Areas of low cloud and fog are a common occurrence along the route between
Fraser Lake and Bulkley Lake, during the fall and spring. Strong westerly winds will
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often result in turbulence between Telkwa and Smithers. In addition, gusty westerly
crosswinds often make landings tricky at Burns Lake.
Weather can change very quickly as cloud invades through the valley at Telkwa.
Precipitation and westerly winds are more frequent at Telkwa and over the town than
at the Smithers Airport. Due to the river just south of the field, fog often lingers, at
times almost randomly drifting across the airport. Fog also forms over the small lake
to the west and can drift over the airport
Smithers to Terrace via Telkwa Pass
NEW HAZELTON
LEGATE
CREEK
NARROW PASS
TERRACE
10,000 FT
7000 FT
5000 FT
3000 FT
SMITHERS
2000 FT
1500 FT
TELKWA PASS
1000 FT
600 FT
COMMON
ROUTES
300 FT
0 SEA LEVEL
Map 4-48 - Smithers to Terrace via Telkwa Pass
The Telkwa Pass route is the most direct route between Smithers and Terrace.
Westbound it starts just south of Smithers and follows the Telkwa River up to the
spectacular Telkwa Pass and then follows the Zymoetz or Copper River to where it
joins the Skeena just east of Terrace.
The route through Telkwa pass is another of BC’s masterpieces during good weather, with hanging glaciers on both sides of the valley. At the pass the valley floor rises
to almost 3,000 feet ASL. The peaks on either side of the pass range from 8,000 to
8,500 feet and are part of the north-south range which separates the wetter coastal
region from the drier interior region.
When westbound through the Telkwa Pass with low cloud or freezing levels, sudden snow squalls or rapidly lowering cloud can reduce ceiling and visibility to near
zero almost instantly. The low cloud or snow will often make dead-end passes appear
to be the main route. This pass should be avoided in any weather conditions likely to
obscure the route. Note that poor conditions in the pass are usually not observable
from either Telkwa or Terrace.
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CHAPTER FOUR
In summer, strong inflow winds can combine with Katabatic winds from the glaciers to cause significant mechanical turbulence. Severe turbulence is also reported to
occur during inflow conditions along the ridge, just east of the town of Terrace.
Smithers to Terrace via the Bulkley / Skeena River Valleys and the
Shorter Route via the Kitseguecla Valley
NEW HAZELTON
LEGATE
CREEK
NARROW PASS
TERRACE
10,000 FT
7000 FT
5000 FT
3000 FT
SMITHERS
2000 FT
1500 FT
TELKWA PASS
1000 FT
600 FT
COMMON
ROUTES
300 FT
0 SEA LEVEL
Map 4-49 - Smithers to Terrace via the Bulkley Valley and Kitseguecla Valley
By following the Bulkley River Valley up toward New Hazelton and then following the Skeena River Valley to Terrace, a pilot can avoid flying over any higher terrain. This is the longest of the three routes between Smithers and Terrace and because
of the lower levels involved, is usually the only option when conditions are marginal.
A short cut can be made on this longer route which cuts between the Bulkley and
Skeena Valleys well south of New Hazelton. Though shorter than the full valley route
it is still considerably longer than the Telkwa Pass route, and it still requires climbing
over terrain at 2,200 ft ASL to traverse the Kitseguecla Pass (usually referred to as
Kits Pass).
Both the Bulkley Valley and Kits Pass routes provide relatively good weather routes
to or from Smithers. The mountains to the west act as barriers to the pacific moisture. Thus there is a significant change in the precipitation regime along the Skeena
River. Typical coastal forests change to drier interior varieties just south of Woodcock
(southwest of New Hazelton). Low cloud often extends up the Skeena, with Legate
Creek a known bad spot. The Terrace Airport weather is generally representative of
that up the valley.
This route is sometimes drier and smoother than the Telkwa Pass option, but both
routes can deteriorate quickly near Terrace. Intrusions of low cloud and fog with
inflow winds are the main weather hazard and, although more frequent in winter, can
occur throughout the year. The airport at Terrace is more prone to low cloud due to
its elevation, approximately 500 feet above town.
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BOB
QUINN
When winds aloft at 6,000 feet exceed 30 knots, moderate turbulence is common
throughout the region. Rounding Copper Mountain at the confluence of the Copper
(Zymoetz) and Skeena Rivers, there is often a wall of wind giving sudden and unexpected turbulence. This usually occurs when strong inflow winds are experienced at
the Terrace Airport.
North from Terrace to the Nass River Valley
STEWART
MEZIADIN
MEZIADIN
LAKE
COMMON
ROUTES
SWAN
LAKE
ALYNSH
LAVA LAKE
SAND LAKE
NEW HAZELTON
KITSUM KALUM
LAKE
10,000 FT
7000 FT
5000 FT
3000 FT
TERRACE
2000 FT
1500 FT
1000 FT
SMITHERS
600 FT
300 FT
0 SEA LEVEL
Map 4-50 - North from Terrace to the Nass River Valley
This route follows the Kalum River to Kitsumkalum Lake, then further north up
the valley to Sand Lake and Lava Lake, before opening into the broad east-west Nass
River Valley south of New Alyansh.
This route is prone to low ceilings. Due to the rise in terrain levels north bound, a
southerly flow creates upslope conditions. If even a scattered layer of low cloud below
700 feet is reported at the Terrace Airport when they are reporting a light southerly
wind, the Kalum River Valley is likely socked in up to Kitsumkalum Lake. Low cloud
also tends to plug this route where the same effect occurs just north of Sand Lake.
The broad Nass River Valley, while generally offering good flying conditions, is
subject to low ceilings and poor visibility under a moist southwest flow off the Pacific.
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CHAPTER FOUR
BOB
QUINN
With inflow up the Nass Valley, expect to encounter poor flying conditions just south
of New Aiyansh where the route from Terrace enters the Nass Valley.
New Hazelton to Meziadin via the Kispiox River Valley or the Kitwanga
River Valley
STEWART
MEZIADIN
MEZIADIN
LAKE
COMMON
ROUTES
SWAN
LAKE
ALYNSH
LAVA LAKE
SAND LAKE
NEW HAZELTON
KITSUM KALUM
LAKE
10,000 FT
7000 FT
5000 FT
3000 FT
TERRACE
2000 FT
1500 FT
1000 FT
SMITHERS
600 FT
300 FT
0 SEA LEVEL
Map 4-51 - Hazelton to Meziadin via the Kispiox River Valley or Kitwanga River
Valley to Dease Lake to Watson Lake
The route goes directly north from New Hazelton follows the Kispiox River Valley
to where it joins the Nass River Valley near Swan Lake. The Kitwanga River Valley
route starts north from the Skeena River Valley approximately 20 n. miles west of
New Hazelton and joins the Nass Valley in the same area. The weather along both
the Kispiox River Valley and the Kitwanga River Valley (the route of Highway 37) is
relatively uniform, both topographically and meteorologically. Both routes usually
offer good flying conditions; however, the area just south of Kitwancool Lake (in the
Kitwanga Valley), has been singled out as more prone to low cloud, under a southerly
upslope flow.
Both routes north merge in the Cranberry Junction, Swan Lake area of the Nass
DEASE
LAKE
TELEGRAPH
CREEK
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STIKINE
ISKUT
Valley. Here the Nass Valley continues north to Meziadin Lake. The terrain is generally flat and fairly swampy and the route is BURAGE
susceptibleCREEK
to low cloud.
BOB flowing up the Nass Valley from the
The frequent invasions of pacific moisture
QUINN LAKE
southwest, particularly in the fall, often bring low ceilings and poor visibility.
BELL IRVING RIVER
Radiation fog is common in autumn, but its frequency diminishes as sources of open
water begin to freeze over with the approach of winter. Except in times of dry cold
outflow, low cloud is an almost constant problem in fall and winter.
Meziadin to Stewart
BOWSER LAKE
STEWART
MEZIADIN
PORTLAND
CANAL
MEZIADIN
LAKE
PORTLAND
INLET
COMMON
ROUTES
10,000 FT
7000 FT
5000 FT
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
NEW HAZELTON
300 FT
0 SEA LEVEL
Map 4-52 - Meziadin to Stewart
Though well inland from the coast, Stewart lies just above sea level at the north end
of the long Portland Canal. Between Meziadin and Stewart the Bear River Valley cuts
through the Coast Mountain Range with peaks either side to almost 9,000 feet ASL.
The Bear River Valley (the route of Highway 37A) from Meziadin Lake to Stewart
is spectacular, but, unfortunately, it is often filled with low cloud and fog, as is the
Portland Canal south of Stewart. The Bear Pass itself is also very narrow. Fixed wing
aircraft cannot usually turn around in the pass and, especially when west bound,
should be confident of conditions at the west end of the pass before entering. When
there are strong inflow or outflow winds, turbulence should be expected, especially
between American Creek and Meziadin,
Weather observation at Stewart should be examined for an idea of conditions near
Meziadin.
153
TELEGRAPH
CREEK
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DEASE
LAKE
Page 154
STIKINE
154
CHAPTER FOUR
ISKUT
Meziadin to Bob Quinn Lake
BURRAGE CREEK
BOB
QUINN LAKE
BELL IRVING RIVER
NINGUNSAW PASS
COMMON
ROUTES
10,000 FT
7000 FT
BOWSER LAKE
5000 FT
3000 FT
STEWART
2000 FT
MEZIADIN
PORTLAND
CANAL
MEZIADIN
LAKE
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
PORTLAND
Map INLET
4-53 - Meziadin to Bob Quinn Lake
North of Meziadin the route leaves the Nass Valley and joins the Bell Irving River
Valley north to the Ningunsaw Pass. The Ningunsaw River flows down from the pass
to the northwest to join the Iskut River Valley just west of Bob Quinn Lake.
Low cloud often closes the route just north of the Meziadin Junction, as well as in
the Ningunsaw Pass south of Bob Quinn Lake. Low cloud and fog will frequently
linger in the vicinity of Bowser Lake, especially in theNEW
fall. HAZELTON
The higher terrain across the Nigunsaw Pass, between the Bell Irving River Valley
and the Iskut River Valley, can be a bottleneck along this route due to low ceilings and
poor visibility. This usually occurs in conjunction with an inflow of moist pacific air
up the Iskut River Valley from the coast.
Pilots usually refer to the two road bridges across the Bell Irving River as Bell 1 and
Bell 2, with Bell 1 being the one further south. The valley north of Bell 1 to, and
including the Nigunsaw Pass, is in a very heavy snowfall belt. In winter it has been
know to receive some of the heaviest snowfalls in northwest BC.
Under inflow conditions, low cloud will often plug the Iskut River Valley south of
Bob Quinn Lake and sometimes extend as far north as Burrage Creek, closing the
route west to Wrangell. The low cloud tends to envelop this area before it reaches
Smithers or even Terrace.
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The weather observations at Stewart and Wrangell, though well west of the route,
are the only indicators of weather conditions for the route. Any precipitation usually
closes these northern valley routes.
Bob Quinn Lake to Dease Lake
DEASE
LAKE
TELEGRAPH
CREEK
COMMON
ROUTES
10,000 FT
7000 FT
5000 FT
3000 FT
2000 FT
1500 FT
EDDONTENAJON LAKE
ISKUT
1000 FT
600 FT
KINASKAN LAKE
300 FT
0 SEA LEVEL
BURRAGE CREEK
BOB
QUINN LAKE
BELL IRVING RIVER
NINGUNSAW PASS
Map 4-54 - Bob Quinn Lake to Dease Lake
North of Bob Quinn Lake, the commonly used air route follows the broad flat
Iskut Valley north on a gradual climb past Burrage Creek to Natadesleen Lake. The
route then branches either to the northeast along Highway 37 to Kinaskan and
Eddontenajon Lakes and through Iskut, or north-northwest to meet the Stikine
STEWART
River north
of Telegraph Creek.
MEZIADIN
The route north climbs
over a pass just north of Iskut and then crosses the eastMEZIADIN
west Stikine Valley. OnLAKE
the north side of the valley it once again climbs over a pass
PORTLAND
and drops into the valley at Dease Lake. The more westerly route turns east when it
CANAL
meets the Stikine River and follows its valley and the Tanzilla Valley to Dease Lake.
PORTLAND
INLET
Inflow up the Iskut Valley often leads to upslope low cloud at Bob Quinn Lake and
up the valley to Burrage Creek. The steadily rising terrain along the Iskut route causes ceilings to lower while flying north. Low cloud and fog are frequently encountered
in the vicinity and just north of the higher elevation pass, north of Iskut. The high
pass just south of Dease Lake can be closed by low cloud with little or no cloud in the
surrounding areas. This route is rarely open in marginal conditions.
NEW
HAZELTON
The alternate route from Natadesleen Lake to just
north
of Telegraph Creek, then
northeastward along the Stikine and Tanzilla River Valleys, is lower in elevation and
often avoids the lower cloud and fog blocking the passes.
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CHAPTER FOUR
The weather report from Ketchikan Airport, Alaska can give some idea of the
weather in the region of Meziadin. If there is low cloud at Ketchikan along with a
westerly wind component, both routes are likely closed.
Dease Lake to Watson Lake
WATSON
LAKE
SW
10,000 FT
7000 FT
5000 FT
DEASE
LAKE
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
COMMON
ROUTES
300 FT
0 SEA LEVEL
Map 4-55 - Dease Lake to Watson Lake
ISKUT
Following the Dease River north, this route climbs over a major upslope regime on
the southwest side of the Cassiar Mountains. At the junction of the Cottonwood and
Dease Rivers the route follows Highway 37 north as it climbs over the ridge of the
Cassiar Mountains. Most aircraft, however, follows the much wider Dease River
Valley more to the northeast through the mountains and then curling back to the
northwest and out on to the plateau near Watson Lake.
Low cloud and fog are common over Dease Lake in the autumn. With systems
tracking in from the west and southwest, heavier precipitation and more frequent
cloud are generally experienced in the upslope area on the western side of the Cassiar
Mountains. Also a strong southwest flow aloft can generate significant turbulence to
the lee of higher terrain just north of Dease Lake, in the area between Joe Irwin Lake
and the Cottonwood River.
North of the mountains, near Four Mile River, the floor of the Dease River Valley
becomes wide and flat as it opens into the plateau. Fog and low cloud are common
over these flat lands and right up to Watson Lake, especially in the fall. However, they
occur much less frequently after freeze up.
There is little or no weather information along this route. Any system activity arriving from the Gulf of Alaska will likely cause precipitation in the mountains and close
the route.
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Dease Lake to Teslin
TESLIN
COMMON
ROUTES
PINE LAKE
RANCHERIA
SW
10,000 FT
7000 FT
5000 FT
3000 FT
DEASE
LAKE
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
Map 4-56 - Dease Lake to Teslin
ISKUT
Under fair weather conditions, this section of northwestern British Columbia can
be scenic and beautiful; however, under poor or deteriorating weather conditions, the
daunting reality is that this vast, sparsely populated wilderness area has few airports
or roads and little or no reported weather.
Weather permitting, pilots will sometimes fly from Dease Lake northwest to the
Teslin River Valley, then northward along the Teslin valley to the community of
Teslin. Moisture from the numerous lakes and rivers that dot the Teslin River Valley
contribute to frequent low cloud and fog in the fall and spring. Strong west or southwest winds can generate significant turbulence to the lee of the Coast Mountains and
higher interior terrain.
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CHAPTER FOUR
Atlin north to the Yukon Border
CARCROSS
TAKU ARM
SKAGWAY
COMMON
ROUTES
SW
10,000 FT
7000 FT
ATLIN LAKE
5000 FT
3000 FT
ATLIN
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
Map 4-57 - Atlin north to the Yukon Border
This route follows the highway north along Atlin Lake to the Yukon border or,
alternately, northwest across the pass at Jones Lake to Taku Arm. These large lakes
are subject to strong winds and rough surface conditions.
A strong southerly flow does not generally produce significant turbulence; however, a strong southwest flow will usually result in areas of moderate turbulence to the
lee of higher terrain along the west side of the Atlin Lake, especially near Atlin
Mountain and Mt. Minto.
PINE LAKE
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Northeast British Columbia
FORT NELSON
FORT ST JOHN
10,000 FT
7000 FT
5000 FT
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
Map 4-58 - Northeast British Columbia
Northeast British Columbia is an extension of the Canadian Prairies. As such it
experiences weather which can vary between typical mountain weather to Prairie conditions, depending on the overall flow pattern.
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CHAPTER FOUR
(a) Summer
The best flying occurs in the late spring to early fall months. During this period,
the weather in general is consistently good for flying. The most prolonged periods of
bad weather will often occur when a cold low moves across the Central Interior of
BC, producing an easterly upslope flow over the area. At these times, widespread low
ceilings and prolonged precipitation will occur and at times will last for twenty-four
hours or more.
This area is also marked by frequent thunderstorms which, due to the plateau type
of terrain, often reach their full intensity. While airmass thunderstorms still remain
the predominant type, frontal thunderstorms and nocturnal thunderstorms are common. The typical scenario would see the beginning of thunderstorm activity early in
the afternoon and for it to persist well into the night. Most often the thunderstorms
move toward the northeast and, given the right conditions, their intensity can reach
the severe level. The normal thunderstorm season for both areas is June to August.
Another phenomenon, which is typical to the prairies, is the development of a
low-level nocturnal jet. (Most common in spring and summer months, this feature
occurs on clear nights following strong, gusty winds in the previous afternoon.) As
the sun sets, a low-level temperature inversion forms. The strong winds remain
several hundred feet off the ground while surface winds become calm. In some cases,
the winds just above the ground can be stronger than the afternoon gusts. The inversion will often deepen during the night to as high as 1,000 feet above ground, before
eroding away the following morning.
(b) Winter
Winter flying conditions can be quite variable. The northeast section of British
Columbia is subject to widespread valley cloud only during the early part of the season, usually until the lakes and rivers freeze over completely. Of particular note is the
Peace River near Fort St. John, which can remain open for prolonged periods during
the winter, resulting in low cloud and fog that can move into the airport with little
notice.
Fronts arriving from the coast tend to move across the area and weaken due to subsidence to the lee of the Coastal Mountains. They will; however, still give steady or
intermittent precipitation whose type will vary depending upon local temperatures.
Accumulation of precipitation is usually light in comparison to the coast. In situations
where warm air is overruning entrenched cold air on the surface, status cloud tends to
form, often becoming both widespread and reluctant to lift. Patchy freezing rain or
drizzle occurs on occasion during the winter months, usually developing east of the
mountains, especially near the arctic front when it lies along the Continental Divide.
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During winter, strong areas of high pressure form in the very cold air over Alaska,
the Yukon and the northern end of the Mackenzie River Valley. This cold arctic air
moves southeastward across the prairies. Depending on the strength of the arctic
front, winds can shift abruptly to the northwest following the frontal passage and be
gusty for several hours. This, coupled with local snowfalls, can produce blizzard conditions. Once an arctic ridge is established over the area, widespread clear, cold weather will prevail except for some localized problems with ice fog.
The southern half of this region can experience Chinook conditions, similar to
those developing across southern Alberta and northern Montana. The effect is often
most pronounced and occurs with greatest frequency close to the Rockies, near
Chetwynd. When an arctic high-pressure system lies to the east of the Rockies, an
intense shear zone can develop as strong southwesterlies aloft blow over surface based
easterly winds. Across the shear zone there is usually a very sharp temperature inversion. In some cases, surface temperatures can be twenty degrees colder than those
found several hundred feet above ground. In such cases, even in clear air, frost can
form on the plane, particularly on the cockpit windows, while ascending into the
warmer air aloft. Chinooks seldom occur in the Fort Nelson area as the arctic air is
usually well entrenched. Some of the sharpest low level temperature inversions occur
over Fort Nelson in winter, sometimes as much as a 30 degree change in less than one
thousand feet. This can cause significant fog or frost on exterior surfaces on ascent.
Pilots have reported parallax errors on descent. As you lower through the inversion,
the line of sight becomes distorted, with the view appearing to shift and magnify.
The strong southwesterly flow aloft that produces the Chinook conditions also
produces lee waves and associated turbulence. However, given that the mountains are
slightly lower and the range is considerably wider, these lee waves possess less energy
than those found over Southern Alberta. Despite this, the ride will often be very
rough, up to approximately 8,000 feet, if winds across the mountains exceed 25 knots.
This lee wave activity does not generally extend north of 59 degrees north latitude
due to the fact that the mountains lose their linear characteristic.
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CHAPTER FOUR
(c) Local effects
Dawson Creek to Fort Nelson
STEAM BOAT
FORT NELSON
NELSON
RIVER
PROPHET
RIVER
MILLIGAN
HILLS
SIKANNI
CHIEF
WONOWON
BEATTON
RIVER
10,000 FT
FORT ST. JOHN
7000 FT
5000 FT
3000 FT
2000 FT
1500 FT
CHETWYND
1000 FT
DAWSON CREEK
Map 4-59 - Dawson Creek to Fort Nelson
600 FT
COMMON
ROUTES
300 FT
0 SEA LEVEL
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There are two commonly used routes between Dawson Creek and Fort Nelson.
The more easterly one follows the railway line via Beatton River and runs mostly
across open plains. The more westerly route via Highway 97 climbs onto a ridge at
Wonowon, runs north past Sikanni Chief and then drops back to the plain south of
Fort Nelson at Prophet River.
The gently rising terrain 10 to 15 miles south of Fort St. John will at times become
shrouded in low cloud. Morning fog, which often forms along the Peace River Valley,
tends to be more frequent at Fort St. John than surrounding airports. The fog usually extends to just north of the Fort St. John Airport.
North of Wonowon, the route following Highway 97 runs along the rising ground
to the southwest of the open plains. This section is much more prone to low cloud
when there is a low-level flow from the northeast quadrant, as these winds cause orographic lift along the upslope terrain. As a result, the high ground between Wonowon
and Prophet River may be obscured while Fort St. John and Fort Nelson both report
clear conditions.
Hills to the east and mountains to the west form a natural lowland area along the
more easterly route via the Beatton River and the railway to Fort Nelson. Many pilots
choose this route when the surface winds are from the north to east quadrant, to avoid
the conditions of upslope flow further west.
The Fort Nelson Airport is in a hollow at the confluence of three rivers and, thus,
experiences more frequent fog than surrounding areas. It also tends to have lighter
winds than the surrounding region. However, expect moderate turbulence in the area
when the airport reports strong westerly winds.
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CHAPTER FOUR
Fort Nelson to Watson Lake
10,000 FT
WATSON LAKE
7000 FT
5000 FT
LOWER POST
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
COMMON
ROUTES
LIARD RIVER
TOAD RIVER
SUMMIT LAKE
STEAM BOAT
SIFTON
PASS
FORT NELSON
Map 4-60 - Fort Nelson to Watson Lake
There are two routes commonly used between Fort Nelson and Watson Lake. The
one following the Alaska Highway runs west from Fort Nelson into the hills and
climbs over a pass at Steamboat of 3,500 feet ASL and then another of over 4,600
feet just west of Summit Lake. It then winds its way through the hills over two lower
passes at about 3,600 feet, before joining the Liard River Valley.
The other route follows a northwesterly heading out of Fort Nelson along the Fort
Nelson River to where it meets with the Liard River. Pilots then follow the river to
Liard River crossing and the highway to Watson Lake. This route avoids the higher
passes and is the preferred route when the higher terrain is obscured. Though the preferred route in marginal weather, it passes through very narrow valleys east of Liard
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Hot Springs. As it is difficult to turn around in these valleys, pilots should not enter
them if they cannot see through them.
Along the highway route, the weather between Steamboat and Liard River often
bears little resemblance to that at either Fort Nelson or Watson Lake. If low cloud is
present at Fort Nelson or Watson Lake, this route is almost certainly closed. Even
with clear skies at both ends, the route may be impassable. Some of the worst weather is typically encountered between Summit Lake and the pass west of Toad River.
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Chapter 5
Airport Climatology
British Columbia
(a) Abbotsford
10,000 FT
7000 FT
ABBOTSFORD
5000 FT
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
Abbotsford is located in the Fraser Valley, approximately 30 nautical miles eastsoutheast of Vancouver. The river valley in which the airport lies is quite broad at this
point, with tree-covered peaks ranging from 3,000 to 4,000 feet ASL within 8 to 15
miles northeast into the southeast. Abbotsford International Airport sits on a slight
rise of land just to the southwest of the city. Northeast of the airport, across the TransCanada Highway, is Sumas Mountain that rises to approximately 3,000 feet.
To the west of the airport is a rolling terrain containing agricultural and urban
areas. To the east is a flat plain known as Sumas Prairie, which was formed from an
old lake bottom and is now primarily used for agriculture.
Abbotsford
Wind Frequency by Direction
Summer
Abbotsford
Winter
N
N
35%
30%
25%
NW
20%
NW
NE
25%
20%
15%
15%
10%
10%
5%
5%
E
W
SW
SE
S
NE
%Calm 20.2
W
E
SW
SE
S
%Calm 16.2
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CHAPTER FIVE
The winter winds show a strong bias to blowing either from the northeast to east,
or from the south to southwest. Northeast winds are very common and can be attributed to a cool katabatic flow that comes out from the eastern end of the Fraser Valley
and curls around Sumas Mountain. Typically, this wind will only be in the 5 to 10
knots range except in the case of very strong outflow conditions. If it is snowing or
there is dry snow on the ground, the winds will produce blizzard-like conditions over
the Sumas Prairie with the main core of strong winds passing just to the southeast of
the airport. During these events, the winds will typically be northeasterly 10 to 20
knots at the airport. When low pressure systems or fronts approach the South Coast,
there is a tendency for these winds to take on a more easterly direction and increase
in strength. The south-southwest winds most often occur behind frontal systems as
they move eastward into the interior. Other directions can occur but they are infrequent and tend to be light.
The summer winds show the same bias for northeast and southwest direction as the
winter winds. However, in this case the most common direction is south to southwest.
This is a sea breeze that often sets up in the mid-late afternoon. Frequently in the
order of 10 to 20 knots, it will also have slightly stronger gusts. This sea breeze tends
to die down in the mid-evening and by midnight the northeasterly katabatic wind
will have re-asserted itself. Even the passage of a summer front will show little change
in the bias to certain wind directions. Other directions can occur but they are infrequent and tend to be light.
The occurrence of low ceilings and visibility in the winter months is almost uniform for any given hour, somewhere between 14 to 17 percent. This is because the
major cause of low conditions is weather systems moving across the area, which have
no diurnal pattern. Fog can be a problem at times, especially if it advects up from the
Bellingham area in Washington but, for the most part, the most common cause of low
ceilings is the frequent and prolonged periods of rain.
Summer tends to be the best time of year at Abbotsford. While cloud ceilings of
2,000 to 4,000 feet can be fairly common, below VFR conditions are rare. For the
most part, they are related to passing weather systems with a slight component due to
radiation fog.
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Frequency of ceilings below 1000 feet and/or visibility below 3 miles
Abbotsford
Frequency (%)
20
Summer
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10 11 12 13
14 15 16 17
18 19
20 21 22
23
Time of Day (UTC)
Abbotsford
20
Frequency (%)
BC-E
Winter
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10 11 12 13
14 15 16 17
18 19
20 21 22
23
Time of Day (UTC)
(b) Castlegar
10,000 FT
7000 FT
5000 FT
3000 FT
CASTLEGAR
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
Located in southeastern British Columbia, Castlegar is located in a narrow valley
on the east bank of the Columbia River, approximately 2 nautical miles southeast of
the city of Castlegar. The Columbia River runs north to south and joins the Kootenay
River about 3/4 mile north of the airport. Other communities in the area are Trail (11
miles southwest), Rossland (14 miles southwest) and Nelson (17 miles northeast).
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CHAPTER FIVE
The terrain immediately surrounding the airport is that of a relatively flat bench
that falls away to the Columbia River, in the west, and to the Kootenay River, in the
north. However, beyond this, the area is hilly, mountainous and thickly wooded. The
bases of the mountains are just over 1/2 nautical mile to the east and just under one
mile to the west. Significant local elevations are Sentinel Mountain, which rises
initially as a 2,500-foot cliff, 1-1/2 miles north-northeast of the runway, then slopes
to a peak elevation of 5,645 feet ASL about 6 miles from the airport; Siwash
Mountain, 7,600 feet ASL, 7-1/2 miles to the northeast; and Mount Mackie, 7,100
feet, 7-1/2 miles to the west-southwest.
Castlegar
Winter
Castlegar
Summer
N
N
20%
25%
NW
20%
NW
NE
15%
15%
10%
10%
5%
5%
E
W
SW
SE
E
W
SW
SE
%Calm 27.0
S
NE
%Calm 29.0
S
The winds at Castlegar show strong channelling by the terrain, both during the
summer and winter. From the graphs, it is evident that the predominant winds are
either from the north or south, about 20 percent of the time for each direction. The
wind can alter slightly in northwest or southeast directions, with the southeast wind
more common in the winter as it moves up the Columbia River Valley. All other
directions are rare. It is worth noting that calm winds occur on an average of 28 percent of the time, summer and winter.
Low ceilings and visibility can be a problem in Castlegar during the winter. Like
most of the valleys in the Southern Interior, the presence of water makes valley cloud
a common phenomenon. However, with a pulp-and-paper plant along the Columbia
River to the northwest and a large smelter operation in Trail to the south, the additional moisture and condensation nuclei make low ceilings a persistent problem.
Below VFR conditions occur on an average of 20 to 25 percent for most hours of the
day, rising to a peak of almost 35 percent at 1500 UTC, which coincides with the time
of maximum cooling. Interestingly enough, the occurrence of ceilings less than 2,500
feet and/or visibility less than 5 miles ranges between 35 and 58 percent, depending
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on the time of day. This means that Castlegar, with its high landing and takeoff limits,
can be closed to commercial traffic for prolonged periods while local pilots are flying
VFR under the cloud deck.
The summer is a much more pleasant story. The valley tends to be dry and hot
much of the summer. As such, the occurrence of low ceilings and visibility is very low,
less than 3 percent, for most hours. There is a slight peak of around 7 percent at 1500
UTC, and most often this is smoke trapped below the nocturnal inversion. This tends
to be a short-lived problem and has disappeared by mid-morning.
Castlegar
35
Summer
Frequency (%)
30
25
20
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10 11 12 13
14 15 16 17
18 19
20 21 22
23
Time of Day (UTC)
Castlegar
35
30
Frequency (%)
BC-E
Winter
25
20
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10 11 12 13
14 15 16 17
Time of Day (UTC)
18 19
20 21 22
23
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CHAPTER FIVE
(c) Cranbrook
10,000 FT
7000 FT
CRANBROOK
5000 FT
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
Cranbrook Airport is situated on a rolling plateau, approximately 5-1/2 nautical
miles north-northwest of the city of Cranbrook, in the southwest BC Interior. The
only other urban centre in the area is Kimberley, 9 miles to the west-northwest.
Two main rivers run past the airport, the St. Mary River and Kootenay River. The
St. Mary River runs in a general easterly direction and passes one mile south of the
airport, while the Kootenay River runs in a general north-northeast to south-southwest direction and passes approximately 5 miles to the northeast. The St. Mary River
joins the Kootenay River 6-1/2 miles east of the airport.
Within 3 miles of the airport, the terrain is slightly rolling, then becomes quite mountainous. The Hughes Range, in which Mount Fisher rises to 9,337 feet ASL, is 11 miles
east-northeast and dominates the area to the north and east. The McGillivra Range,
with a peak height of 7,240 feet, lies 10 miles to the southeast while the Moyie Range
dominates the western quadrant, rising to 5,800 feet within 10 miles of the airport.
Cranbrook
Winter
Cranbrook
Summer
N
N
15%
20%
NW
15%
NW
NE
NE
10%
10%
5%
5%
W
E
SW
SE
W
E
SW
SE
%Calm 26.9
S
%Calm 46.3
S
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Cranbrook is not a windy location. During the winter months it is calm almost 46
percent of the time and the winds are less than 10 knots almost 90 percent of the
time. When the wind does blow it has a strong preference for directions ranging from
southeast to west. Winds from the south to west occur most often a few hours ahead
and behind frontal systems as they move across the interior. The southeast wind, out
of the Rocky Mountain Trench, is often a result of cold air invading from Alberta.
This cold air tends to move through the Crowsnest Pass into the Trench. The cold air
then tends to flow northwestward filling the Trench.
Cranbrook is slightly windier in the summer, mainly due to local convection, but
even so it is calm 27 percent of the time. Like winter, it has a preference for winds
from southeast to west. These winds tend to be largely a product of local pressure differences, as well as the movement of frontal systems through the area.
Cranbrook is seldom a problem due to low ceilings or visibility. During the summer, the probability of IFR conditions is less than 2 percent. During the winter, the
probability is typically around 10 percent rising to a peak of 20 percent during the
morning hours. This increase in probability is a mix between system weather, often
snow, and the occurrence of local fog that makes its way onto the airport.
Cranbrook
Frequency (%)
20
Summer
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10 11 12 13
14 15 16 17
18 19
20 21 22
23
Time of Day (UTC)
Cranbrook
20
Frequency (%)
BC-E
Winter
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10 11 12 13
14 15 16 17
Time of Day (UTC)
18 19
20 21 22
23
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CHAPTER FIVE
(d) Fort Nelson
10,000 FT
7000 FT
5000 FT
3000 FT
FORT NELSON
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
Fort Nelson is situated in the northeast corner of B.C. Straddling the Alaska
Highway, it is a favourite stopping point for aircraft following the highway towards
Alaska or returning from there.
Situated in a flat valley, the airport is exposed to the full extremes of the Canadian
climate. During the winter, arctic air pools in the valley and can hold temperatures at
subzero values long after the higher terrain warms up. The sharp inversions these cold
pools form are a favourite haunt for widespread low stratus, fog and freezing drizzle.
In the summer, the area is also subject to upslope flow resulting in widespread nearIFR conditions. Even when the weather is nice, thunderstorms are a frequent visitor.
On the plus side, Fort Nelson is not a windy airport. While the winds can blow
from almost any direction, about 70 percent of the time they remain at below
10 knots.
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Fort Nelson
Summer
Fort Nelson
Winter
N
N
20%
20%
NW
NW
NE
15%
15%
NE
10%
10%
5%
5%
W
E
SW
SE
W
E
SW
SE
%Calm 20
S
%Calm 41
S
It is the low ceilings that offer the greatest difficulty to aviation. While no more frequent than Fort St. John, they can be extremely prolonged at Fort Nelson, going on
for days at a time. This serves as a major problem for aircraft wishing to transit
through the area.
Fort Nelson
25
Summer
Frequency (%)
20
15
10
5
0
0
1
2
3
4
5
6
7
8
9 10 11
12 13 14 15 16 17
18
19 20 21 22
23
Time of Day (UTC)
Fort Nelson
25
20
Frequency (%)
BC-E
Winter
15
10
5
0
0
1
2
3
4
5
6
7
8
9 10 11
12 13 14 15 16 17
Time of Day (UTC)
18
19 20 21 22
23
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CHAPTER FIVE
(e) Fort St. John
10,000 FT
7000 FT
5000 FT
3000 FT
FORT ST. JOHN
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
Fort St. John lies to the east of the Rocky Mountains and shares a climate similar
to northwest Alberta. Situated on terrain that slopes gradually towards the east, it is
subject to warm, dry summers and cold winters.
The winds of summer show three distinctive axes. The northerly winds occur quite
frequently when a cold front moves southward through Alberta. Confined by the
Rockies, the cooler air has little choice but to flow southwards along the mountains.
The most common wind, the southwesterly, is a ‘chinook-type’ flow. Downslope off
the Rockies, these winds tend to be strong, gusty and warm. They also usually
provide excellent flying weather. The wind that offers the greatest problem is the east
or southeast wind. This wind is upslope and often brings with it widespread low ceilings and visibility in precipitation and mist.
Fort St. John
Summer
Fort St. John
Winter
N
N
30%
30%
25%
NW
25%
NW
NE
15%
15%
10%
10%
5%
5%
W
E
SW
SE
S
NE
20%
20%
%Calm 6.0
W
E
SW
SE
S
%Calm 9.0
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The winter winds show a stronger preference for the cold northerlies, usually associated with the arctic front. Another common wind is the warm southwesterlies. The
result is that the entire area is subject to wide swings in temperature. The northerlies
can be a problem if the cold air is moving slowly. In such a case, Fort St. John can
receive a prolonged period of low ceilings and visibility in snow. Meanwhile, the east
or southeast winds continue to be a problem in that there is a river to the southeast
of the airport that remains open most winters. Fog banks form over this water and the
wind carries it into the airport.
The following ceiling and visibility graph for Fort St. John confirms these periods
of below VFR operations but also reveals that the occurrence is not all that frequent.
25
Fort St. John
Summer
Frequency (%)
20
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10 11
12 13
14 15 16 17
18 19
20 21 22
23
Time of Day (UTC)
25
Fort St. John
20
Frequency (%)
BC-E
Winter
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10 11
12 13
14 15 16 17
Time of Day (UTC)
18 19
20 21 22
23
177
BC-E
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CHAPTER FIVE
(f) Kamloops
10,000 FT
7000 FT
5000 FT
KAMLOOPS
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
Kamloops Airport lies on the floor of the Thompson River Valley, a deep, narrow
valley oriented in an east-west line. The site is about midway between the southern
and northern branches of the westerly flowing river. The centre of the city of
Kamloops lies approximately 5 miles to southeast, on the southern banks of the
Thompson River.
From the valley floor, hills rise rapidly to heights of 3,000 to 4,000 feet ASL, which
in turn give way to even higher ridges and peaks beyond 9 miles of the airport.
Porcupine Ridge, 19 miles to the north-northwest, rises to 6,100 feet ASL; Mount
Lolo, about 13-1/2 miles to the northeast, attains 5,500 feet; Chuwheels Mountain,
nearly 11 miles south-southwest of the airport, is 6,233 feet; and, Greenstone
Mountain, 9 miles to the southwest, rises to 5,883 feet ASL.
The hillsides up to an elevation of 3,000 feet ASL are grassed, with little or no tree
growth. The higher slopes and plateau are where pine and spruce forests thrive.
Kamloops
Winter
Kamloops
Summer
N
N
30%
25%
NW
35%
30%
25%
NW
NE
20%
15%
10%
15%
10%
5%
5%
W
E
SW
SE
W
E
SW
SE
%Calm 22.5
S
NE
20%
%Calm 26.4
S
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The winds at Kamloops Airport show the effects of the east to west orientation of
the Thompson River Valley. During the winter months, the wind is easterly almost
35 percent of the time or calm 26 percent of the time. These easterly winds are a product of katabatic winds that develop along the local slopes most nights, converge in the
valley bottom and flow towards Kamloops Lake. Normally these winds are only 5 to
10 knots, but can increase ahead of approaching frontal systems or with very strong
drainage of arctic air towards the coast. Westerly winds occur less often and tend to
be strong and gusty in the wake of an arctic front.
The terrain continues to influence the direction in the summer but convection gives
a larger variation in wind direction. As in winter, the valley has a preference for an
easterly drainage wind overnight and during the morning. In the afternoon, convection
begins to mix the upper winds down to the surface causing the direction of the wind
to shift to westerly. Also, frontal systems which move across the interior at
regular intervals, bring wind shifts from east to west with their passage. Some of the
strongest westerly winds will occur in this case. It is worth noting that, on occasion,
strong subsidence winds aloft are brought down to the surface as south-to-southwest
(190-230 degrees true) winds. These winds are not only quite strong but definitely
abrupt in their onset.
The Thompson River Valley tends to be hot and dry in the summer, creating a need
for widespread irrigation. As such, below VFR conditions almost never occur.
The same cannot be said for the winter months. While the area is fairly dry, snow
does occur from time to time and valley cloud is all too ready to put in an appearance.
Cold air tends to get entrenched in the valley and with it comes valley cloud. Below
VFR conditions occur about 10 percent of the time throughout the day with the
probability rising to near 15 percent in the early morning, the time of maximum cooling. These statistics do not tell the whole story. The probability of ceilings below
2,500 feet and/or visibility below 5 miles is about 30 to 35 percent and only during
the afternoon, the time of maximum heating, does the probability drop to around
20 percent. When you take into account some of the local elevations, this high occurrence of low cloud should make any pilot wary.
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CHAPTER FIVE
Kamloops
15
Summer
12
Frequency (%)
180
11/12/05
9
6
3
0
0
1
2
3
4
5
6
7
8
9
10 11 12 13
14 15 16 17
18 19
20 21 22
23
Time of Day (UTC)
Kamloops
15
12
Frequency (%)
BC-E
Winter
9
6
3
0
0
1
2
3
4
5
6
7
8
9
10 11 12 13
14 15 16 17
18 19
20 21 22
23
Time of Day (UTC)
(g) Penticton
10,000 FT
7000 FT
5000 FT
3000 FT
PENTICTON
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
Penticton Airport is located in the Okanagan Valley, a deep, north to south valley
in the Southwest Interior of British Columbia.
With such marked terrain influence, it should come as no surprise that most of the
time the winds are either northerly or southerly.
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In the summer months, the winds are predominately thermally driven and result in
a frequency distribution that is split between north and south. During the day, sunny,
hot conditions tend to prevail especially in the south end of the valley. The strong
convection produced by this heating eventually carries the predominately southerly
winds aloft to the surface, resulting in strong southerly winds that persist into the
evening. Overnight, the valley cools but remains warmer in the south, resulting in a
weak trough of low pressure. Air begins to drain towards this low resulting in northerly winds that will persist into the morning. On the other hand, if the upper winds are
not southerly or very light, a northerly wind will persist for the entire day.
Penticton
Summer
Penticton
Winter
N
N
40%
NW
30%
NW
NE
30%
10%
W
E
SE
S
NE
20%
20%
10%
SW
40%
%Calm 20
W
E
SW
SE
S
%Calm 17
The winter wind pattern is largely driven by pressure gradient. Strong lows and
frontal systems moving onto the coast result in significant pressure falls over the
province, especially the Central Interior. Strong southerly winds develop through the
southern valleys ahead of these systems, especially the trailing cold fronts, then
change to a northerly flow in its wake. It should be noted that this wind change will
not occur if the pressures to the north of the airport remain lower than those to the
south. Northerly winds also occur when cold arctic air spreads into the Southern
Interior and drains towards the coast and Washington.
The effects of a drier climate are very apparent in the ceiling and visibility charts
for Penticton. Below VFR conditions occur only about 10 percent of the time.
Seasonally, these occurrences are almost strictly confined to the winter period and are
related to the formation of valley cloud.
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CHAPTER FIVE
Penticton
14
Summer
12
Frequency (%)
182
11/12/05
10
8
6
4
2
0
0
1
2
3
4
5
6
7
8
9
10 11
12 13 14 15 16 17
18 19 20 21 22
23
Time of Day (UTC)
Penticton
14
12
Frequency (%)
BC-E
10
Winter
8
6
4
2
0
0
1
2
3
4
5
6
7
8
9
10 11
12 13 14 15 16 17
18 19 20 21 22
23
Time of Day (UTC)
(h) Port Hardy
10,000 FT
7000 FT
5000 FT
3000 FT
PORT HARDY
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
Port Hardy Airport is located on the northeastern end of Vancouver Island. For aircraft flying north or south along the coast, this airport along with Prince Rupert are
key destinations. The airport lies on the western shore of Queen Charlotte Strait with
the northern end of the Vancouver Island mountains just to the west of the airport.
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Like most other BC airports, the season of the year has a strong influence on the
wind. In the summer, the most frequent winds tend to be easterlies that occur just
ahead of frontal systems and northwesterlies that occur with its passage. The pressure
falls due to the heating over the southern end of the island. This induces a sea breeze
that begins as a northwesterly wind over Northern Queen Charolette Strait in the late
afternoon, spreads southward and then slackens to light winds near midnight.
Port Hardy
Summer
Port Hardy
Winter
N
N
40%
NW
30%
NE
30%
10%
10%
W
E
SE
S
NE
20%
20%
SW
40%
NW
%Calm 21
W
E
SW
SE
S
%Calm 14
Winter at Port Hardy is a wet, windy time of year. Strong lows and frontal systems
move onto the North Coast, generating frequent east to southeast winds up Queen
Charlotte Strait and over the airport. While the airport can experience periods when
the winds are light or calm, it is worth noting that over 25 percent of the time in the
winter the winds are in excess of 10 knots. Very frequently, the winds are much
stronger over Queen Charlotte Strait and these winds often move onto the airport a
few hours ahead of an approaching frontal system.
Port Hardy experiences below VFR flying conditions any time of the year. During
the summer flying is generally good; however, fog banks are often located over
Queen Charlotte Sound and often move onto the airport during the early morning.
The worst flying conditions occur during the period of late fall through winter, into
early spring. Strong weather systems move up onto the coast resulting in widespread rain and low ceilings. Between the major systems there is a tendency for poor
weather to move into Port Hardy just prior to sunrise and often persist into the afternoon. On the plus side, when the strong southeast winds are blowing the airport tends
to remain just above the IFR category.
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CHAPTER FIVE
35
Port Hardy
30
Frequency (%)
184
11/12/05
Summer
25
20
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10 11
12 13 14 15 16 17
18 19 20 21 22
23
Time of Day (UTC)
35
Port Hardy
30
Frequency (%)
BC-E
25
Winter
20
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10 11
12 13 14 15 16 17
18 19 20 21 22
23
Time of Day (UTC)
(i) Prince George
10,000 FT
7000 FT
5000 FT
PRINCE GEORGE
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
Prince George is the largest city in the Central Interior. The weather here is
extremely variable, as the major storm tracks off the Pacific prefer to lie across this
area. Cloudy skies and precipitation tend to be common, yet in the summer the days
can be hot and sunny with the strongest thunderstorms anywhere in the province.
Conversely, the arctic front can sweep across the area bringing clear, frigid conditions.
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The wind pattern is fairly uniform across the entire year. Southerly winds tend to
develop ahead of weather systems and then become northerly in their wake.
Prince George
Summer
Prince George
Winter
N
N
40%
40%
NW
30%
NW
NE
30%
20%
20%
10%
10%
W
E
SW
SE
%Calm 16
S
NE
W
E
SW
SE
S
%Calm 17
During the summer months, the airmass tends to be fairly convective. Thus, below
VFR weather does not occur often except when major systems, such as a cold low,
move across the area.
The same cannot be said for the winter. A major influence on weather in the Prince
George area is the local forestry mills. There are three forestry mills located to the
north of the airport. The combination of the moisture pumped out by these mills and
the condensation nuclei provided ensure that low cloud and fog is a real problem. The
normal trend is to see the low cloud and fog move into the airport during the night
and persist well into the morning. These conditions tend to be episodic in that poor
weather will redevelop night after night, until the wind or the airmass changes.
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CHAPTER FIVE
Prince George
30
Summer
25
Frequency (%)
186
11/12/05
20
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10 11 12
13 14 15 16 17
18 19 20 21 22
23
Time of Day (UTC)
Prince George
30
25
Frequency (%)
BC-E
20
Winter
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10 11 12
13 14 15 16 17
18 19 20 21 22
23
Time of Day (UTC)
(j) Prince Rupert
10,000 FT
7000 FT
5000 FT
3000 FT
PRINCE RUPERT
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
Prince Rupert Airport is located on an island just to the west of the city of Prince
Rupert. With the Coast Mountains lying right along the shores of the sea, Prince
Rupert is considered by many to be the cloudiest, wettest city in British Columbia.
The winds at Prince Rupert are for the most part light. Even in the winter months
when vigourous weather systems sweep into the North Coast, the airport winds are
less than 10 knots 75 percent of the time.
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The wind patterns at Prince Rupert show a strong seasonal variation. In the summer months, two predominate wind directions prevail. The marked southeast wind
occurs ahead of approaching weather systems. The west winds occurs behind the
fronts and with sea breezes.
Prince Rupert
Summer
Prince Rupert
Winter
N
N
40%
NW
30%
30%
20%
10%
W
E
SE
S
NE
20%
10%
SW
40 %
NW
NE
%Calm 16
W
E
SW
SE
S
%Calm 12
In winter, the strong low-pressure systems that move up across the Queen
Charlottes make their presence felt. Almost one-third of the time the winds will blow
strongly from the south to southeast.
Prince Rupert Airport is a tough airport even for commercial aircraft. The combination of weather systems crossing the coast in winter, as well as low cloud from the
ocean moving onshore, keeps Prince Rupert below VFR conditions 30 to 35 percent
of the time. Even in the summer, prolonged periods of rain and fog banks often form
in the area or move ashore resulting in below VFR conditions.
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CHAPTER FIVE
Prince Rupert
40
Summer
35
Frequency (%)
188
11/12/05
30
25
20
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10 11 12
13 14 15 16 17
18 19 20 21 22
23
Time of Day (UTC)
Prince Rupert
40
35
Frequency (%)
BC-E
30
Winter
25
20
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10 11 12
13 14 15 16 17
18 19 20 21 22
23
Time of Day (UTC)
(k) Sandspit
10,000 FT
7000 FT
SANDSPIT
5000 FT
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
The hamlet of Sandspit, the Gateway to Gwaii Haanas National Park, is located
on the The Queen Charlotte Islands, off the north BC coast. The Queen Charlotte
Islands consist of several islands of which two of the largest are Graham Island, in the
north, and Moresby Island, in the south. Between Graham and Moresby Islands is a
narrow channel known as Skidegate Channel that broadens into Skidegate Inlet on
the east side.
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The only settlement on Moresby Island, Sandspit lines both sides of the low-lying
spit of land at the eastern end of Skidegate Inlet. This spit also protrudes out into
Hecate Strait, which lies between the mainland coast and the Queen Charlotte
Islands. Sandspit Airport is located at the end of this spit, the runway elevation a mere
20 feet above sea level.
The area immediately around the airport is relatively flat with a sandy, grassy covering. To the west, the terrain becomes forested and begins to rise into a set of ridges
that rise above 1,000 feet, 10 miles to the southwest of the airport.
Sandspit
Summer
Sandspit
Winter
N
N
30%
25%
NW
30%
25%
NW
NE
20 %
15 %
10%
10%
5%
5%
W
E
SW
SE
W
E
SW
SE
%Calm 10.5
S
NE
20%
15%
%Calm 9.7
S
The winds at Sandspit are a product of only a limited number of influences. The
southeast winds occur ahead of approaching low pressure systems. Beginning fairly
light, these southeast winds will increase as they are channelled between the
Coast Mountains and the Insular Mountains. At the peak of a storm, winds can be
impressive at Sandspit with the winds being even stronger over Hecate Strait. It has
been noted by forecasters that if the isobars on a surface weather map are oriented
more northwest-southeast than north-south, then convergence will cause the winds
to be strongest over the Sandspit.
The westerly wind is a channelled wind through Skidegate Inlet. Sandspit Airport
will frequently show strong, gusty westerly winds behind a front that will last for
several hours, then begin to diminish. These westerly winds will, however, persist
until the high pressure ridge that is following the frontal system moves across the
area. All other directions can occur but are infrequent.
Sandspit is fully exposed to all of the major systems that move through the area.
With rain common in these systems, low cloud ceilings occur frequently. At the same
time, the waters around the Charlottes are prone to low cloud and fog whenever
warm air moves over the relatively cold water. As a result, below VFR conditions can
189
11:32 PM
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CHAPTER FIVE
be expected around 15 percent of the time in the winter and 8 percent of the time in
the summer. The nicest conditions in winter occur during times of outflow winds
through the mainland inlets, although snow showers can be a problem over Hecate
Strait and along the eastern shores. In summer, the best flying weather occurs when
strong ridges of high pressure become fixed over the area.
Sandspit
20
Frequency (%)
190
11/12/05
Summer
15
10
5
0
1
2
3
4
5
6
7
8
9
10 11 12 13
14 15 16 17
18 19
20 21 22
23
Time of Day (UTC)
Sandspit
20
Frequency (%)
BC-E
Winter
15
10
5
0
1
2
3
4
5
6
7
8
9
10 11 12 13
14 15 16 17
Time of Day (UTC)
18 19
20 21 22
23
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(l) Terrace
10,000 FT
7000 FT
5000 FT
3000 FT
TERRACE
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
Terrace Airport is located on a plateau across the river just south of town. Despite
being located inland from the coast, its weather is all too often a dramatic clash
between the moist air from the coast and drier air from the interior.
Situated at the junction of several valleys, the wind pattern at Terrace Airport
shows the strong influence of topography and pressure gradient.
During the summer months, the winds are quite variable and blow from almost all
quadrants. The most prominent wind is a southerly that blows out of the Kitimat
River Valley from Douglas Channel. This wind usually occurs ahead of an approaching frontal system. This southerly wind also occurs in the summer when there is a
thermal trough inland and a ridge of high pressure along the coast. The other prominent wind is a northerly drainage wind out of the Kitsumkalem River Valley. The
winds do blow easterly and westerly over the airport but are seldom strong.
Terrace
Summer
Terrace
Winter
N
N
50
50
40
NW
NW
NE
40
20
20
10
10
W
E
SW
SE
S
NE
30
30
%Calm 16
W
E
SW
SE
S
%Calm 20
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CHAPTER FIVE
The influence of cold air and falling pressure along the coast ahead of a strong
weather system is evident in the winter wind pattern at Terrace. North winds are very
common ahead of a weather system. These winds also occur in outflow situations and
can result in near blizzard conditions. Just ahead of an approaching frontal system, the
winds will often switch to a southerly from the Kitimat River Valley as warm air
begins to move into the area from the coast.
Like Prince Rupert, Terrace can be a very difficult airport for weather forcasting.
During the winter, upslope conditions off the coast mixing with cold air from the
interior will produce low cloud and poor visibility in rain or snow, depending on the
temperature. During the changeover from snow to rain, mixed rain and snow or freezing rain can make flying into this area treacherous. Even when the precipitation
stops, all too often radiation fog forms, rapidly producing prolonged IFR conditions.
July and August are the best flying months in this area. Low cloud off the ocean
can still be a problem but, in general, conditions remain VFR.
Terrace
40
Summer
35
Frequency (%)
192
11/12/05
30
25
20
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10 11
12 13 14 15 16 17
18 19
20 21 22
23
Time of Day (UTC)
Terrace
40
35
Frequency (%)
BC-E
30
Winter
25
20
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10 11
12 13 14 15 16 17
Time of Day (UTC)
18 19
20 21 22
23
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(m) Vancouver
10,000 FT
7000 FT
VANCOUVER
INTERNATIONAL
AIRPORT
5000 FT
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
Vancouver Airport is located on an island at the mouth of the Fraser River. With
the Fraser Valley extending off to the east and the Strait of Georgia lying in a rough
northwest-southeast line just to its west, the winds are strongly influenced by topography and season.
During the summer months, the winds are predominately east or west. The west
wind tends to be a sea breeze while the east wind is a drainage wind coming out of
the Fraser Valley. Minor frontal systems do move across the area resulting in a southeast wind ahead of the front changing to a northwest wind in its wake. Seven percent
of the time the winds are calm and 85% of the time the winds are less than 10 knots.
Only on the rare occasion do winds exceed 20 knots.
Vancouver
Summer
Vancouver
Winter
N
N
50%
40%
NW
50%
40%
NW
NE
30%
20%
20%
10%
10%
W
E
SW
SE
W
E
SW
SE
%Calm 7
S
NE
30%
%Calm 10.0
S
The winter winds show a similar pattern but are stronger in strength. The easterly
wind is especially noticeable. Added to the normal easterly drainage wind, that occurs
193
11:32 PM
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CHAPTER FIVE
almost every night at Vancouver Airport, is the enhanced easterly winds ahead of
approaching coastal lows and arctic outflow winds that occur once or twice per winter.
The summer tends to be the best time of the year for recreational flying. The only
real periods of below VFR conditions occur when low sea stratus from Juan de Fuca
Strait gets drawn into the area.
The period late fall through winter into early spring is the most difficult time of the
year. Strong weather systems move up onto the coast resulting in widespread rain and
low ceilings. Between the weather systems, there is a noticeable peak near 1700 UCT
which is just after local sunrise in the winter. Frequently at this time, fog or status
slides into the airport and does not break up until the sun rises higher into the sky
and some heating occurs.
40
Vancouver
Summer
35
Frequency (%)
194
11/12/05
30
25
20
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10 11
12 13 14 15 16 17
18 19
20 21 22
23
Time of Day (UTC)
40
Vancouver
35
Frequency (%)
BC-E
30
Winter
25
20
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10 11
12 13 14 15 16 17
Time of Day (UTC)
18 19
20 21 22
23
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(n) Victoria
10,000 FT
7000 FT
5000 FT
3000 FT
VICTORIA INTERNATIONAL
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
Victoria International Airport is situated on Vancouver Island, at the northern end
of the Saanich Peninsula, 14 nautical miles north of the city of Victoria, and just to
the west of the small city of Sidney. The airport is in close proximity to bodies of
water on three sides; Saanich Inlet, one mile west; Satellite Channel, 3 miles to the
north; and Haro Strait, 1-1/2 miles to the east. The airport is readily affected by
marine influences.
The terrain around the airport is relatively flat except for Mount Newton that rises
to 1,000 feet, some 3-1/2 miles to the south-southwest. However, just beyond
Saanich Inlet, the Insular Mountains of Vancouver Island rise to 3,000 feet from the
southwest to northwest direction.
Victoria
Summer
Victoria
Winter
N
N
30 %
25 %
NW
30 %
25 %
NW
NE
20 %
15 %
15%
10%
10 %
5%
5%
W
E
SW
SE
W
E
SW
SE
%Calm 12
S
NE
20%
%Calm 8.0
S
Victoria Airport is not noted for its winds; in fact, about 90 percent of the time the
winds are less than 10 knots. In winter, the winds at Victoria Airport are almost
equally distributed around the compass, except for a strong preference for a westerly
195
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CHAPTER FIVE
direction. This wind is a katabatic flow off of the Insular mountains, occurs most
nights, and tends to be light, 5 to 10 knots. Winds from other directions also tend to
be light, except for southeast winds which occur ahead of weather systems crossing
Vancouver Island, and southwest winds that occur in the wake of these systems. With
the passage of the cold front, a strong, gusty flow through the Strait of Juan de Fuca
moves through Victoria Harbour and down across the airport, but it only persists for
a few hours.
Summer winds are also mainly light and show a similar preference for the katabatic
westerly flow. However, there is an increased occurrence of east to southeast winds.
These winds are largely sea breeze in nature and occur during the afternoon and
early evening.
Victoria is drier than most airports along coastal BC because it lies in the “rain
shadow” of the Olympic Mountains. As such, the low cloud in precipitation tends to
take longer to form ahead of an approaching frontal system and lifts to above 1,000
feet fairly quickly in its wake. Victoria Airport, being nearly surrounded by water, is
quite susceptible to stratus and fog that have formed over the sea. Banks of stratus and
fog move onto the airport, particularly those which formed over Pat Bay to the west.
In such a case, the katabatic wind tends to carry it onto the airport near 0900 UTC,
and it can persist to late morning, 1700 UTC or so.
During the summer, the occurrence of low ceilings and visibility is quite low, less
than 5 percent. Like winter, occasional fog off the ocean will move into the airport
but tends to dissipate quickly after the sun rises.
During the winter, Northeast “Strait effect” winds can bring snow showers and low
visibilities as cold air funnels out of the mainland inlets and valleys. Some say that part
of the reason that Victoria received much more snow during Storm ’96 was due to
this local effect.
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Victoria
25
Summer
Frequency (%)
20
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10 11
12 13
14 15 16 17
18 19
20 21 22
23
Time of Day (UTC)
Victoria
25
20
Frequency (%)
BC-E
Winter
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10 11
12 13
14 15 16 17
18 19
20 21 22
23
Time of Day (UTC)
(o) Williams Lake
10,000 FT
WILLIAMS LAKE
7000 FT
5000 FT
3000 FT
2000 FT
1500 FT
1000 FT
600 FT
300 FT
0 SEA LEVEL
Located in the Central Interior, Williams Lake airport lies 4 nautical miles northeast of the town of Williams Lake. The only other urban centres in the immediate
area are 150-Mile House, just under 7 miles to the southeast, and Glendale, about
5 miles to the west-southwest.
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CHAPTER FIVE
The airport is situated on the Fraser Plateau, approximately 7 miles east of the
Fraser River. The Fraser River runs in a north-south line and is quite narrow. Four
miles to the south of the Airport is Williams Lake, which is about 1/2 mile wide and
nearly 4 miles long.
The surrounding countryside is hilly but only lightly wooded. The airport sits on
one of the highest elevations in the area although a ridge, elevation just over 3,900
feet, lies 11 miles to the northeast of the airport.
Williams Lake
Winter
Williams Lake
Summer
N
N
40%
35%
3 0%
25%
NW
20%
NW
NE
NE
25%
15%
20%
15%
10%
5%
10%
5%
W
EW
SW
SE
E
SW
SE
%Calm 29.3
S
%Calm 30.8
S
The summer winds at Williams Lake are quite benign, being calm almost 30 percent of the time and less than 10 knots almost 90 percent of the time. When wind
does occur, it shows a strong bias to being either from the northwest or southeast.
This is usually the result of passing frontal systems and is strongly influenced by the
orientation of the local terrain.
Winter is not much different. Winds continue to remain calm almost 30 percent of
the time, and less than 10 knots, around 86 percent of the time. Of note is the
preference for southeast winds over all other directions. The arctic front takes up a
favoured position near Prince George for much of the winter. Ahead of frontal
systems, the southeasterly winds frequently develop along the Fraser River and often
reach values of 20 gusting to 30 or more knots. However, behind the front, the cold
northwesterly winds tend to be confined behind the arctic front.
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Williams Lake
25
Summer
Frequency (%)
20
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10 11 12 13
14 15 16 17
18 19
20 21 22
23
Time of Day (UTC)
Williams Lake
25
20
Frequency (%)
BC-E
Winter
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10 11 12 13
14 15 16 17
Time of Day (UTC)
18 19
20 21 22
23
199
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Glossary of Weather Terms
anabatic wind - a local wind which blows up a slope heated by sunshine.
advection - the horizontal transportation of air or atmospheric properties.
air density - the mass density of air expressed as weight per unit volume.
air mass - an extensive body of air with uniform conditions of moisture and temperature in the horizontal.
albedo – the ratio of the amount of solar radiation reflected by a body to the
amount incident on it, commonly expressed as a percentage.
anticyclone - an area of high atmospheric pressure which has a closed circulation
that is anticyclonic (clockwise) in the Northern Hemisphere.
blizzard - a winter storm with winds exceeding 40 km/h, with visibility reduced by
falling or blowing snow to less than one kilometre, with high windchill values
and lasting for at least three hours. All regional definitions contain the same
wind speed and visibility criteria but differ in the required duration and temperature criterion.
cat’s paw – a cat paw-like, ripple signature on water given by strong downdrafts or
outflow winds. A good indication of turbulence and wind shear.
ceiling - either (a) the height above the surface of the base of the lowest layer of
clouds or obscuring phenomena (i.e. smoke) that hides more than half of the
sky; (b) the vertical visibility into an obstruction to vision (i.e. fog).
chinook - a warm dry wind blowing down the slopes of the Rocky Mountains and
over the adjacent plains.
clear air turbulence (CAT) - turbulence in the free atmosphere not related to convective activity. It can occur in cloud and is caused by wind shear.
clear icing - the formation of a layer or mass of ice which is relatively transparent
because of its homogeneous structure and smaller number and size of air
spaces; synonymous with glaze.
climate - the statistical collection of long-term (usually decades) weather conditions at a point; may be expressed in a variety of ways.
cold front - the leading edge of an advancing cold air mass.
convection - atmospheric motions that are predominately vertical, resulting in the
vertical transport and mixing of atmospheric properties.
convergence - a condition that exists when the distribution of winds in a given area
is such that there is a net horizontal inflow of air into the area; the effect is to
create lift.
cumuliform - a term descriptive of all convective clouds exhibiting vertical
development.
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GLOSSARY
cyclone - an area of low atmospheric pressure which has a circulation that is
cyclonic (counterclockwise) in the Northern Hemisphere.
deepening - a decrease in the central pressure of a pressure system; usually applied
to a low. Indicates a development of the low.
deformation zone - an area in the atmosphere where winds converge along one axis
and diverge along another. Where the winds converge, the air is forced upward
and it is in these areas where deformation zones (or axes of deformation as they
are sometimes referred to) can produce clouds and precipitation.
disturbance - applied loosely: (a) any small-sized low pressure system; (b) an area
where the weather, wind, and air pressure show signs of cyclonic development;
(c) any deviation in flow or pressure that is associated with a disturbed state in
the weather; and (d) any individual circulatory system within the primary circulation of the atmosphere.
divergence - a condition that exists when the distribution of winds in a given area is
such that there is a net horizontal outflow of air from the area.
downdraft - a small scale downward current of air; observed on the lee side of large
objects that restrict the smooth flow of air or in or near precipitation areas
associated with cumuliform clouds.
downburst - an exceptionally strong downdraft beneath a thunderstorm usually
accompanied by a deluge of precipitation.
filling - an increase in the central pressure of a pressure system; applied to a low.
Föhn wind (foehn wind)- a warm dry wind on the lee side of a mountain range,
whose temperature is increased as the wind descends down the slope. It is
created when air flows downhill from a high elevation, raising the temperature
by adiabatic compression.
front - a surface, interface or transition zone of discontinuity between two adjacent
air masses of different densities.
Fujita Scale – a scale used to rate the intensity of a tornado by examining the damage caused by the tornado after it has passed over a man-made structure
(see Table 1).
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Table 1 - The Fujita Scale
F-Scale
Number
Intensity Phrase
Wind Speed
(kts)
Type of Damage Done
FO
Weak
Tornado
35-62
Some damage to chimneys; breaks branches off trees;
pushes over shallow-rooted trees; damages sign boards.
F1
Moderate
Tornado
63-97
The lower limit is the beginning of hurricane wind speed;
peels surface off roofs; mobile homes pushed off foundations
or overturned; moving autos pushed off the roads; attached
garages may be destroyed.
F2
Strong
Tornado
98-136
Considerable damage. Roofs torn off frame houses;
mobile homes demolished; boxcars pushed over; large trees
snapped or uprooted; light-object missiles generated.
F3
Severe
Tornado
137-179
Roof and some walls torn off well constructed houses;
trains overturned; most trees in forest uprooted
F4
Devastating
Tornado
180-226
Well-constructed houses leveled; structures with weak
foundations blown off some distance; cars thrown and
large-object missiles generated.
F5
Incredible
Tornado
227-285
Strong frame houses lifted off foundations and carried
considerable distances to disintegrate; automobile-sized
missiles fly through the air in excess of 100 meters; trees
debarked; steel re-inforced concrete structures badly damaged.
funnel cloud - a tornado cloud or vortex cloud extending downward from the parent cloud but not reaching the ground.
gust - a sudden, rapid and brief increase in wind speed. In Canada, gusts are
reported when the highest peak speed is at least 5 knots higher than the
average wind and the highest peak speed is at least 15 knots.
gust front - the leading edge of the downdraft outflow ahead of a thunderstorm.
high - an area of high barometric pressure; a high pressure system.
hurricane – an intense tropical weather system with a well defined circulation and
maximum sustained winds of 64 knots or higher. In the western Pacific, hurricanes are called “typhoons,” and similar storms in the Indian Ocean are called
“cyclones” (see Table 2 for hurricane intensitites).
Table 2 - Saffir-Simpson Hurricane Scale
Category
#
Sustained Winds
(kts)
Damage
1
2
3
4
5
64-82
Minimal
83-95
Moderate
96-113
Extensive
114-135
Extreme
>155
Catastrophic
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GLOSSARY
icing - any deposit of ice forming on an object.
instability - a state of the atmosphere where the vertical distribution of temperature
is such that a parcel displaced from its initial position will continue to ascend.
inversion - an increase of temperature with height - a reversal of the normal
decrease of temperature with height.
isothermal layer - equal or constant temperature with height.
jet stream - a quasi-horizontal stream of wind concentrated within a narrow band;
generally located just below the tropopause.
katabatic wind - downslope gravitational flow of colder, denser air beneath the
warmer, lighter air. Also known as “drainage wind” or “mountain breeze”.
Strength can vary from gentle to extremely violent winds.
knot - a unit of speed equal to one nautical mile per hour.
lapse rate - the rate of change of an atmospheric variable (usually temperature) with
height.
lee wave - any stationary wave disturbance caused by a barrier in a fluid flow; also
called mountain wave or standing wave.
lightning - any and all forms of visible electrical discharge produced by a thunderstorm.
low - an area of low barometric pressure; a low pressure system.
meridional flow – airflow in the direction of the geographic meridians, i.e. southnorth or north-south flow.
meteorology - the science of the atmosphere.
mixed icing - the formation of a white or milky and opaque layer of ice that
demonstrates an appearance that is a composite of rime and clear icing.
occluded front - a front that is no longer in contact with the surface.
orographic - of, pertaining to, or caused by forced uplift of air over high ground.
outflow - a condition where air is flowing from the interior land area through
mountain passes, valleys and inlets onto the coastal areas; used most commonly
in winter when cold Arctic air spreads onto the coastal area and adjoining sea.
overrunning - a condition when warm air overtakes or is lifted by colder denser air.
parcel - a small volume of air, small enough to contain uniform distribution of
meteorological properties, and large enough to remain relatively self-contained
and respond to all meteorological processes.
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plow wind - usually associated with the spreading out of a downburst from a thunderstorm; a strong, straight-line wind in advance of a thunderstorm that often
results in severe damage.
precipitation - any and all forms of water particles, whether liquid or solid, that fall
from the atmosphere and reach the surface.
quasi-stationary front - a front that is stationary or nearly so; commonly called
stationary front.
ridge - an elongated area of relatively high atmospheric pressure; also called ridge
line.
rime icing - the formation of a white or milky and opaque granular deposit of ice
formed by the rapid freezing of supercooled water droplets.
saturation - the condition in the atmosphere where actual water vapour present is
the maximum possible at the existing temperature.
shower - precipitation from cumuliform cloud; characterized by suddenness of
beginning and ending, by rapid changes in intensity, and usually by rapid
changes in the appearance of the sky.
squall - essentially gusts of longer duration. In Canada, a squall is reported when
the wind increases by at least 15 knots over the average speed for a duration of
at least 2 minutes and the wind reaches a speed of at least 20 knots.
squall line - a non-frontal line or narrow band of active thunderstorms.
stability - a state of the atmosphere where the vertical distribution of temperature
is such that a parcel will resist displacement from its initial position.
stratiform - term descriptive of clouds of extensive horizontal development; flat,
lacking definition.
stratosphere - the atmospheric layer above the tropopause; characterized by slight
increase in temperature from base to top, very stable, low moisture content and
absence of cloud.
subsidence - the downward motion of air over a large area resulting in dynamic
heating.
supercooled water - liquid water at temperatures below freezing.
thunderstorm - a local storm invariably produced by a cumulonimbus cloud, and
always accompanied by lightning and thunder.
tornado - a violently rotating column of air, shaped from a cumulonimbus cloud,
and nearly always observed as “funnel-shaped;” other names are cyclone and
twister.
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GLOSSARY
tropopause - the transition zone between the troposphere and the stratosphere;
characterized by an abrupt change in lapse rate.
troposphere - the portion of the earth’s atmosphere from the surface to the
tropopause; characterized by decreasing temperature with height and
appreciable water vapour. Often referred to as the weather layer.
trough - an elongated area of relatively low atmospheric pressure; also called
trough line.
trowal - a trough of warm air aloft; related to occluded front.
turbulence - any irregular or disturbed flow in the atmosphere.
updraft - a localized upward current of air.
upper front - any frontal zone which is not manifested at the surface.
virga - water or ice particles falling from a cloud, usually in wisps or streaks, and
evaporating completely before reaching the ground.
warm front - the trailing edge of retreating cold air.
weather - the instantaneous conditions or short term changes of atmospheric
conditions at a point; as opposed to climate.
wind - air in motion relative to the earth’s surface; normally horizontal motion.
wind direction - the direction from which the wind is blowing.
wind speed - rate of wind movement expressed as distance per unit time.
wind shear - the rate of change of wind direction and/or speed per unit distance;
conventionally expressed as vertical and horizontal wind shear.
zonal wind - a west wind; conventionally used to describe large-scale flow that is
neither cyclonic or anticyclonic; also called zonal flow.
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Table 3: Symbols Used in this Manual
Fog Symbol (3 horizontal lines)
This standard symbol for fog indicates areas where fog is frequently observed.
Cloud areas and cloud edges
Scalloped lines show areas where low cloud (preventing VFR flying) is known to
occur frequently. In many cases, this hazard may not be detected at any
nearby airports.
Icing symbol (2 vertical lines through a half circle)
This standard symbol for icing indicate areas where significant icing is relatively
common.
Choppy water symbol (symbol with two wavelike points)
For float plane operation, this symbol is used to denote areas where winds and
significant waves can make landings and takeoffs dangerous or impossible.
Turbulence symbol
This standard symbol for turbulence is also used to indicate areas known for
significant windshear, as well as potentially hazardous downdrafts.
Strong wind symbol (straight arrow)
This arrow is used to show areas prone to very strong winds and also indicates the
typical direction of these winds. Where these winds encounter changing topography
(hills, valley bends, coastlines, islands), turbulence, although not always indicated,
can be expected.
Funnelling / Channelling symbol (narrowing arrow)
This symbol is similar to the strong wind symbol except that the winds are constricted
or channeled by topography. In this case, winds in the narrow portion could be very
strong while surrounding locations receive much lighter winds.
Snow symbol (asterisk)
This standard symbol for snow shows areas prone to very heavy snowfall.
Thunderstorm symbol (half circle with anvil top)
This standard symbol for cumulonimbus (CB) cloud is used to denote areas
prone to thunderstorm activity.
Mill symbol (smokestack)
This symbol shows areas where major industrial activity can impact on aviation
weather. The industrial activity usually results in more frequent low cloud and fog.
Mountain pass symbol (side-by-side arcs)
This symbol is used on aviation charts to indicate mountain passes, the highest point
along a route. Although not a weather phenomenon, many passes are shown as they
are often prone to hazardous aviation weather.
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APPENDIX
Appendix
Skagway
•
• Dease Lake
Mucho Lake
• Fort Nelson
• Ware
• Stewart
• Sikanni Chief
Langara •
Prince Rupert
•
•
Terrace
Sandspit •
• Smithers
• Germansen Landing
• Fort St John
• Mackenzie
• Burns Lake
BRITISH COLUMBIA
Cape St James
•
• Prince George
Mcinnes Island •
• Bella Coola
Port Hardy
• Quesnel
• Puntzi Mountain
•
Williams Lake
•
• Blue River
Clinton •
Whistler
•
Tofino •
Vancouver •
•
• Kelowna
Hope
•
Victoria •
Revelstoke
•
Kamloops
Oliver
•
Cranbrook
•
•
Castlegar
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MAP INDEX
Chapter 4 area maps
Teslin
•
Atlin • 158
• Watson Lake
157
156
• Dease Lake
164
155
• Fort Nelson
154
103
151
Langara •
104
Sandspit
146 • Ware
162
• Sikanni Chief
• Stewart
153
Prince Rupert
•
• Germansen Landing
• Fort St John
144
• Dawson Creek
• Mackenzie
150 152
149
•
• Smithers
105 Terrace
•
• Burns Lake
148
102
143
98
Cape St James
•
Mcinnes Island •
• Prince George
140
• Bella Coola
• Quesnel
138
• Puntzi Mountain 142
Williams Lake •
139
96
100
Port Hardy •
137
88
91
Tofino •
Whistler •
87
85
Vancouver• 82
80
Victoria •
Blue River
•
Clinton
141
136
•
132
113-114
130
126 •
• Kamloops
Banff
•
Golden
Revelstoke
110
•
125
129
83
• Kelowna
Hope 115
118
116
•
123
Oliver
121
Cranbrook
•
• 124
•
Castlegar
209
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Page 2
11/07/05

The Weather of British Columbia

Transcription

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